What explains the disparity of outcome in people who join the military?

You're assuming that the dramatically segmented and stratified experience is the "exact same service", which is patently not true.

There's a world of difference between a black officer who managed a supply line, a white non-comp who maintained aircraft, and a queer rifeman who witnessed half his unit die to an IED. And that's ignoring rape, gang infiltration, religions discrimination, and plain ol "was your CO a jerk or awesome?"

I suspect that mental health and social support at home play huge roles here.

I would assume it also has to do with your assignment and how much/if any combat you’ve experienced and injuries you sustained.

My Dad was drafted into Vietnam - infantry - and was wounded. He was only there ~ 6 months, and tried to go to university on the GI Bill, but for awhile couldn’t stand being in that type of environment immediately after returning home. PTSD/survivor’s guilt, etc, were too much.

Later on in life his injuries prevented him from being able to work. This, combined with rising medical debt, left us in a bad spot.

What you’re exposed to when you serve and when you served also come into play - PTSD is taken more seriously now, as are the effects of things like Agent Orange. I’m not sure if the VA is better or worse from, say, 20 years ago, but that could also be a factor.

Not sure if this helps to answer your question, but that’s my personal anecdote.

They are all given a clean slate somewhat and are exposed to the exact same environment

OK, try actually imagining this "clean slate" and "same environment". Imagine yourself, right now, going to boot camp alongside someone who's lived a life of relative privilege with lots of educational and exercise opportunities - someone better educated, fitter, even taller than you. Imagine yourself alongside someone who lived a life lacking in privileges - a crappy or unfinished education, not particularly healthy, shorter than you.
Imagine how you and those two hypothetical people tackle the different obstacles this "same environment" gives you - imagine going through an actual obstacle course with them. How easy or hard does each person find each obstacle? Will a short person find climbing a rope or wall as easy as a tall person? How about balancing on a log, crawling under wire, or jumping over a ditch?
A "clean slate" isn't any kind of reset of their abilities and experiences. "The same environment" inherently means it challenges their individual abilities differently. They get different outcomes because they were different people to begin with, so react differently to their experiences, like everywhere else in life.

In the military there are two separate classes, officers and enlisted. Officers had at least a bachelor's degree and the military is a much different experience for them. They come in day as leaders, are paid significantly more, and have more opportunities to make lifelong connections with powerful people.

Enlisted are treated like blue collar workers, the grunts. Just in their day to day jobs enlisted people are going to have more wear and tear on their bodies. Take battle experience out of it, just as is, officers have a much better path forward.

Fraternization is illegal between officers and enlisted which basically makes it two segregated classes.

Enlisted folks also tend to come from lower income families, so while they are giving more opportunities than staying in their home town it can still be difficult for them to advance much higher.

In short, you have a large mixture of people from different ways of life with different education levels. Some take advantage of stuff like the GI Bill or other opportunities,. others don't. Also some give their bodies to the extent that they can't really function at the same levels on the outside.

Sample bias. Any advertising, campaigning, fawning and celebrating are the exceptions. You are exposed to the "success stories" exponentially more through media thanks to government and corporate forces despite the successes being exponentially rarer than the failures: suicides, mental health disorders, divorces, denied medical care by VA, insufficiency of college fund programs, underemployment, etc. The coverage Success Stories get as the 1% or whatever, dwarfs the failures which are the 99%. This reversed representation explains why they may be perceived as equally likely, which is confusing.

The answer is sample bias; deliberately misleading. After all, who is going to sign up if they could see reality represented? Most would just work fast food--same crappy outcomes, fewer bullets.

Three things stand out to me:

1. The disparity between military and civilian certifications. If you're a mechanic on a specialised machine, it's easy to become a DOD contractor on that same platform. If you're an officer, you can say you've managed X people. If you're a medic, your scope of practice is command-by-command. You might work above an LPN level but leave the military with an NREMT EMT-B certification at most which qualifies you for a minimum wage job way below your scope of practice. There are only a handful of slots for advanced schools that give you any worthwhile certification, and the paperwork required to become an LPN ($55k/year~) is very difficult to amass. If you're a machine gunner, you spent 4+ years mastering a skill that doesn't transfer to any civilian job and you can only sell yourself as a whipped horse with a broken back.

2. Military culture itself is traumatic. It's closer to feudalism than it is anything in the civilian world. There's a rigid hierarchy and set of standards for every aspect of your life. You have a pathological obsession with being 15 minutes early because your lord can have you arrested if you don't do everything perfectly. You're supposed to embrace the toxicity of every part of your day, much less being a cog in the demon machine that hates you as much as it does its victims, and tie your self-worth to that scripted performance. The outlet for dealing with any of that is alcoholism and/or smoking. You can't afford or make the time for civilian therapy, while military therapists are inquisitors that can have you arrested or end your career.

3. There's no blueprint for civilian life. The allure of military life is that you know exactly what every expectation is. You can turn your brain off because you know what to wear and how to wear it, what to do and how to do it, and how doing A will result in B for your career. Your chain of command is a line of narcissistic older siblings and parents strictly directing you down that path. You have that stick incentive of being arrested if you violate any part of the carrot plan. All of a sudden you're removed from that very traumatic environment and it's replaced by a much more abstract system where nobody follows the rules you're conditioned into.

Maybe you make that deal with the devil knowing you're a good fiddler and you get a golden fiddle for it. If you didn't explicitly do that for that reason and get the right paperwork for that goal, you leave with nothing and probably hate the field you were trained for but not meaningfully certified in.

Family and social supports when you get out. There is a lot of adjustment to a world where you have to make your own way and decisions. Having support lets you make some mistakes and recover. Even if that is just getting a wrong job, or not finding a place to live fast enough.

Military service is traumatising, but can build up a nest egg of funds, so to speak. If your trauma doesn't turn into a self-reinforcing cycle, you're in a better position than most in our society (materially, not morally). It lends itself to falling in one extreme or another.

That said, I imagine there's a decent number of people who get regular jobs in the middle that you never hear about. Just the people at the bottom get the whole "they sacrificed their mental health and their life, why can't we help them" signal boost

I don't have data to support it, but I'd imagine that the job role within the military can make a big difference. Were you an officer, with a college degree, doing a lot of IT work and never deployed? You're probably gonna be fine.

Were you an enlisted undez who scraped rust, or were deployed and suffering from PTSD? It's gonna be a much harder time.

I keep seeing posts like this, and I have to wonder if OP is a kid or if they just have no real life experience. Do homeless people stay homeless in your mind?