Why do most prep school kids go to liberal arts colleges after graduating?

I've noticed that many people have the belief that the brightest of the class go to ivy league schools, but I know otherwise. Any ideas why?Why do most prep school kids go to liberal arts colleges after graduating?
It's not most as ivies, Stanford, Duke and a few others are well represented, but top LACs are considered on par, just a different experience by those in the know. There are kids who want the small seminars and small communities. The Williams, Swats and Amhersts of the world are harder to get into than the bottom ivies, Cornell and non Wharton Penn. After small classes and close relationships with teachers all through school many just don't want the impersonal big U.