Because a) they were there first and b) it's human decency to not want to force someone else to be uncomfortable. I don't know why you'd want to force another family's experience to be less than it could be so you can squeeze next to an unfamiliar child in order to save 20 seconds of your day.
It's not hostility. it's caution. There are actual examples on this very thread of abusive things happening to kids on rides. It does happen. A theme park like WDW is a PERFECT place for predators, and you've got to know that. There are people -- a very low percentage, I'm sure, but an actual one -- of sexual predators who go to WDW because of the opportunity it presents. And you can't expect parents to ignore that unpleasant reality.
But some do. Some go to WDW for that specific purpose. And as much as you might resent the implication, even you concede the possibility. So why not just take away the chance to make someone uncomfortable? You talked about seats on busses -- yeah, you ought to think about it before you sit down to a kid on a bus. Yeah, parents ought to show caution before they send kids off to bathrooms unaccompanied. It may be unfortunate reality of society, but it is a reality that single men can be seen as a threat, and it seems to be to be a reasonable bit of kindness to understand and at least show a bit of compassion to that reality and adjust behavior accordingly. And sometimes that means standing on a bus, and sometimes it means waiting for the next car. I think WDW would be well served to make it policy that solo riders are allowed to sit alone if they choose.