People get big cats for pets when they are little cats and then when they get to be big they either escape or are released. It is becoming illegal now in most places but there is still a big black market in them and will be as long as people are willing to pay for them.
I've known a couple of people that had big cats and both eventually lost them. the good news is that they are not much of a danger to people but then they also don't have much fear of people like a wild born animal would. I've seen a big cat once here and we don't have any native big cats so I know that they are out there. I mostly feel sorry for them. They are free but not naturally wild and I don't think that most of them live long.
Mountain lions are seen here in some LA neighborhoods from time to time. Parts of LA border wilderness areas and that is where they show up. They've been seen a few times in my neighborhood. And, now we have bears too. Someone who lives not far from me just posted a Ring camera video of a large mountain lion casually walking by on the sidewalk in front of her house at night.