I take transit a lot and many times of the day and while I've seen all of these things, most of the time you see harmless stuff. Mostly it's teens (kids who can't drive yet), doing dumb things like drawing graffiti on a map or putting silly stickers on things. Occasionally someone smelling like booze and weed ends up on the train. Someone who hasn't showered in... days gets on. Sometimes you notice a person who is obviously not fully there and talks to themselves a lot or mumbles and shouts with no pretext. Occasionally you see a person with untreated visible medical conditions (like an abscess). If you have headphones in or have a book in your face, you probably won't even notice what's happening. The numbers back these up as it's statistically much safer to take transit in most places than drive. I do have friends in the suburbs though, so I know there are folks who find safety and comfort in appearances. And for some folks they find genuine solace in their car which is fine.
You don't even _know_ who you're driving next to on a freeway. The driver next to you may be driving home dead tired, "microsleep"ing along the way. They could be very drunk, trying desperately to get home and crawl into bed. Maybe they got fired at their workplace cause they were on meth. Someone driving might have a seizure and lose control of their vehicle. You can just look at traffic crash statistics; the US is the worst developed country for traffic incidents by far. Just because you can't see them doesn't mean their conditions don't actually exist.
Don't get me wrong, I'd love to see America pay actual attention to the folks with these physical and medical conditions, and the fact that we don't is terrible. But stuffing them into cars or forcing them to an underclass where they can't drive isn't the answer. And it's not particularly unsafe to take transit.