Because meeting the characters is a HUGE deal for a lot of smaller kids, and, additionally, experiencing the parks at 4 or 5 is significantly different than experiencing them at 9 or 10. It's magical for kids of that age in a way it can't be for older kids (not saying it isn't still magical for older kids; it's just different). I loved seeing how happy my niece was at 3 when she got to meet and hug Tigger; I'm sure it's infinitely more fulfilling for a parent to get to see their kid have that experience.
Plus, I personally think most of the thrill rides aren't as fun as the non-thrill rides. Stuff like Spaceship Earth, Horizons, the original Journey Into Imagination, Pirates of the Caribbean, etc. are all better than any thrill ride I've ever been on. The more thrill rides Disney builds (especially at the expense of other types of rides), the less interesting Disney World becomes.