My guess is, if you look into it, you’ll find that in most of the cases you are thinking of, per OP, it was the person’s last degree that was the launching point for their big career. Their JD, MBA, MD, whatever.
Many of the “better” and most selective graduate and professional programs include Ivy league universities.
The undergrads do relatively well as a group, IIRC, but most people do not achieve the kind of fame or “top” spot you are referring to.
That they do relatively well as a group is probably in no small part due to the fact that they were admitted to a selective college because there were already indications that they were likely to do well.