I have no idea what it's like to be famous, I had no famous friends growing up, but I did grow up fairly rich as did most of the people who went to the same school as me growing up..
I believe people who are smart are more likely to have smart kids than a couple who is not smart - that's just genetic probability. And people who have made their wealth (as opposed to just being born into it like the royal family) are usually very smart. So I think a small part of it is that yes, their IQ on average (I say on average) is generally slightly higher than someone from a poorer background. This is not me saying all rich people are smart and all poor people are dumb, because I have seen many exceptions, but just a small trend.
Secondly, these kids are given lots and lots of tutoring. Growing up I had tutors for French, Maths and Chemistry. So of course I was going to grow up smarter and do better academically than if I didn't have these advantages, which helped me get into the university I got into (not to mention my highschool was very good and taught me many effective study techniques and how to think for myself). No, my parents didn't bribe anyone (I don't know anyone who got into a university through a bribe), but yes I did have a lot of advantages.
Lastly, a lot of poorer people may be very gifted, but just cannot afford to go to university (unless they get a scholarship). So in this case a person who can afford the tuition but with lower grades is more likely to be accepted into a university than a person with fantastic grades but without a scholarship and who cannot afford to pay the full fees. Luckily nowadays in Europe (not sure about the US) a lot of universities try to compensate for this by offering many scholarships and lowering tuition fees.