There's not much appeal in them honestly. It's just your basic easy to learn, easy to play game that throws rewards at you.
It's phone game tier trash. All of it.
Only difference is that you get to beat on real people, and match making usually ensures you win every other match, on average. Add that you can play with your friends. And let's face it, most things are fun, or more so, if you do it with friends. And you get entertainment of some kind.
Why some people choose to play these forever and ever though... well, it's the same phenomena as that which causes people to rewatch TV series and movies, or reread books and comics.
You know you're going to have a good time to some degree, with little effort, if you do X again.
But with Y which you haven't experienced before, you don't know if it's going to be fun or not. So you go with the sure thing.