Being born into a conservative household can be a hard hurdle to clear. I grew up with the unquestioning belief that the left was straight-up evil (shocker: that was projection) but then moved around a ton and worked alongside a huge diversity of people after highschool cuz I joined the military and didn't have a choice: that exposure was a real shock, but since our brains don't like being wrong, I resisted it for a good while before finally acknowledging that I was acting like a moron and started thinking more critically about politics and what political decisions meant for my community.
Not everyone gets that healthy slap-to-their-senses. Doesn't excuse shit, but that's the 'why'.
It'd be interesting to see some actual political metrics on other service members. The military is always seen as being SOLID red, and while yes it does lean that way, the tiny bubble of the military that was my personal field of view seemed maybe a 60-40 split; and I personally went in red, and separated borderline radical blue. I know at least a handful of others who did the same... no idea if it's always been that way, or if this is a developing trend. Or if I happened to be stationed in an uncharacteristically blue slice of military. /shrug.
This is my exact story to a "t". I grew up in a heavily conservative household, joined the military as a conservative, and 15 years later I'm pretty fucking blue.
I think it's a combination of a few things that does it to us more often than not:
1 - Exposure to people from all walks of life/escaping your "bubble"
2 - Access to tax payer funded social programs that the rest of America desperately fucking needs and going "Why can't taxes do this for EVERYONE!?"
3 - If you've been deployed to certain sections of the world, you see first-hand what unfettered religious extremism can potentially do to a country.
I'm happy to say, that at least for the Air Force, I've run into far, far more Active Duty Dems/Libs than I have any Repubs. Now, when the retired veteran GS employees come in, it's a completely different story. My whole circle save one is fairly left-leaning, and the one who isnt is...fucking weird/all over the place on his stances.
In my experience 20 years ago, it was mostly split between people who didn't care or at least didn't talk politics (the majority) and people who were very loud in thinking a Democrat would reduce the pay of enlisted service members.
I think a lot of your political beliefs start with your parents and people around you, but then are shaped by what happens to you. If you get hit with the hard reality of life in some way (debt, health, etc) it tends to push you to be more progressive/empathetic toward others because you see just how truly cold and cruel the bottom of our society gets treated. If you have coasted through life and have parents and friends supporting you financially and we're lucky enough to get a good job and have relatively good health and such then you may not know the horrors of what can happen if you hit a few rough patches.
Similarly in military service, if you get injured and now have to go to the VA and fight to receive the most basic of medical care you were promised and denied it tends to push you left/progressive because you want things to be better. If you are still in the service or left it relatively undamaged then you could easily still see things from a conservative and right leaning perspective.
When you or people close to you get (denied medical care, denied housing, denied work, pushed into poverty and/or homelessness, used by the system and then discarded like trash) you tend to see things more progressive and want to provide some basic levels of support and humanity and empathy toward others as opposed to exploiting them for profit.
30% of your time being red. 30% of your time coming to your senses. 40% of your time trending towards borderline radical blue.
There is your 60/40 split.
Anecdotally, this was somewhat my experience as well, except my parents didn’t talk politics much and they certainly didn’t show the extreme hate towards the left that fox and rush have since incited. I went more from voting on surface level attributes (speaking ability, apparent warmth, etc) and social pressure, to actually [eventually] looking at policy. I was a registered independent when I joined, but I didn’t know enough about actual politics to understand the details of what I was voting for. I.e., I was going with the flow. It has the same effect at the ballot box though.