In terms of frequency of visits, the main reason I go to the USA is to visit close friends there. That's more to do with where they live than the USA. However I also go to the USA a lot for tourism, and even when I am visiting friends, I generally enjoy it.

Some of the things I like are:
* The huge variety of natural landscapes - forests, mountains, lakes, plains, deserts, coast, all on a scale and variety far greater than near where I live. The most recent natural landscape I went to see in the USA was the California coastal redwood forests, something you just can't see in Northern Europe.
* The big cities with great museums, architecture, an interesting history, often good food (definitely in any of the coastal cities).
* The views of the country as you fly across it - I can't understand anyone who takes a US internal flight and doesn't want a window seat!
* Hotels are usually reliably good (or at least predictable), and fairly cheap outside the most popular destinations.
* Good roads (though the other side of that is you have to drive almost everywhere, so you have to rent a car most of the time to go most places)
* More clothes for tall people than most of Europe, and sometimes technology is cheaper too.
* Usually there is little language barrier, people are usually friendly and give good service in the service industries, and by now I know how the country works so I can get around easily

I've been to a lot of European countries and I'm working my way around Asia (and Australia and NZ), but I still go back to the USA regularly too. I'm not nearly done with it.

Of course there are downsides, and many parts of the USA are places I have no intention of going for fun, but the place is so big with so many things to do, that it's easy to avoid them.