I think the reason people like theme parks is pretty easy to explain.
I wanted to focus on why I love rollercoasters in particular, like the ones at amusement parks without a real theme.
Consider what rollercoasters are, basically massive constructions of wood or steel, that goes through a very in-depth process, and entirely designed to make people have a thrilling experience...nothing else.
To the guest, there's no storyline, no thinking about what it all means, nothing at all except a pure experience for a few minutes that they can't safely get in many other ways.
There are many ways for people to have fun, but IMO rollercoasters are the purest form of providing fun on a large scale.