You don't see it because you happen to prefer Millennium Force, which is perfectly acceptable. Things like Millennium Force...well, you wait in a boring line for up to several hours and then ride a ride just because it's fast and high...that's all right. That's how thrill rides were designed.
Other types of people don't like waiting in a typical queuing area and riding a ride that's just high and fast. Sure, they want it to be all of that, but they want an EXPERIENCE as well. Bigger and faster things come along and the rides that USED to be the biggest and fastest get forgotten along the way. What you remember better are the experiences you have on certain rides. You'll think about Millennium Force in twenty years and you'll remember, "Oh, I waited in line for like, two hours, but I don't remember the ride that well." Whereas other rides have certain memorable qualities...theming. FoF has the UFO you walk into; that's something you're going to remember and think, "Wow, that was pretty cool." Same with the helicopter in IJ:ST.
It's perfectly fine that those aren't the type of rides you enjoy, but surely you can see why others like them?