They can, and do, but when hotels are directly attached to the park or very easily accessible there's a BIG difference. Most people visiting Universal have little to no idea that the hotels are connected, and you don't see a lot of looky-loos wandering around (and I say this as someone who IS one of those people!). I think about at Disney how many people just hop on the Monorail and hotel hop in the middle of the day because they're hot and tired and the lobbies and public areas turn into flop houses of sweaty people. If the hotel is directly attached it just makes it even more attractive for people to camp out-- especially passholders and off duty team members.