Because they have different causes and different solutions.
Car crashes are killing X people; if you insist on treating all car crashes as though they were caused by someone intentionally driving into a crowd--versus inattentive drivers or driving drunk--then you aren't going to be able to solve the problem.
Idunno, making guns a lot harder to obtain would help a lot with both and so would replacing a lot of cops in both poorer neighborhoods and schools with social workers and mental health professionals 🤷
replacing a lot of cops in both poorer neighborhoods
That's a band-aid on a broken leg: useless. Social workers and mental health professionals aren't going to help when the problem is poverty. What are they going to do, make you feel better about not being able to make rent, or getting your shitbox 20yo Toyota repo'd because your hours got cut and you couldn't make the payments? Fix the economics, and the rest largely solves itself.
Social workers and mental health professionals aren't going to help when the problem is poverty
That's where you're wrong. Both social workers and mental health professionals can ABSOLUTELY help you reach out to get the help you need and make you better equipped to deal with and change the system, leading to a reduction in poverty. It's less of a band-aid on a broken leg and more one small but extremely beneficial piece of a huge puzzle.
...And that's what's helping, not the social workers. Ain't nobody got time to be fighting for change when you're working 14 hour days just to be able to make rent.
The social workers aren't directly addressing the problem. The social workers ensure that you don't have to work 14 hour days just to get by at a barely-outside-of-poverty level. Feeling better about yourself isn't going to mean that you don't have to work yourself to utter exhaustion just to get by.
The problem is the system we live in, the system that allows vast inequities in wealth, the system that requires poor and unemployed people in order to keep profitability high for owners. Reducing cops and adding social workers doesn't directly address that, and it's unlikely that it would indirectly address that either. Yes, we should fix our criminal justice system. Yes, people living in poverty should have realistic access to mental health. But that should be because it's the right thing to do, not because that's going to fix the underlying inequities that cause the problems.
I think you're right in what you're saying but not what I think you're implying. The solution to one problem is mental health regulations, the solution to the other is improper use regulations. Which are different, but fall under the same umbrella.
Either way, taking about cars is exactly where I'd take the comparison because it's the only other commonly owned (highly effective) weapon in the us. We require a license to use one and liability insurance to own one, then take that license away if you show that you can't use it properly. That's exactly what if like to see happen to guns.
"Regulation" per se doesn't actually solve the underlying problems, it just masks it. Solutions are things that address the underlying issue. In that case of most violent crime, that's poverty, systemic racism, lack of opportunity, economic inequality, and so on. All of those things would also tend to decrease mental health issues, but going farther and making sure that people could realistically access good mental health help--without fear of loss of rights--would improve outcomes.
Moreover, in the example I used, there were three causes of crashes; there are absolutely, definitely cases of people using a car to intentionally kill pedestrians in indiscriminate attacks. Regulation doesn't prevent that kind of abuse, unless you want to simply, y'know, entirely ban the use of all vehicles. Which would require entirely overhauling all of the infrastructure in the US.
As far as 'school shootings'--which I'm defining as violence targeting schools with the intent of causing mass casualties--you don't have a single solid cause. There's a report--by the US Secret Service, I think?--that tried to nail down factors, and what they came up with was that there wasn't any single thing that most or all of them had in common that wasn't a legal and protected activity, or wasn't common enough that targeting that factor wouldn't have widespread unintended consequences. E.g., if 50% of the students that committed mass murders were bullied, then putting all bullied kids on a watch list would be punishing an enormous number of kids for the actions of a very tiny minority.