I think pretty much every university, including Stanford, the Ivies and other top schools, is overweighted toward students from its home region. A majority of even the highest achieving students tend to stick close to home.
IIRC the percent of Stanford applicants from California is even higher than the 32.9% of matriculated students.
The military service academies (West Point, Annapolis, Air Force) are the only exceptions I can think of, because they have an admissions process that is specifically designed to ensure representation from all states.