If I recall correctly, something like 80% of students go to a school within 100 miles of where they grew up, and 90% within 500 miles. I assume those percentages are lower for top students, but even a lot of them will prefer to stick close to home in the end (and certainly many parents will prefer that).
I believe that Stanford and similar schools try pretty hard to have national student bodies (with obviously some internationals as well), but still they can only admit from among those who apply, and they get a disproportionate number of applications from their regions. It wouldn’t surprise me if yield is also higher among admits from their regions.
That 33% from California was more like high 30s only a few years ago if I recall correctly, so the trend is toward a more national student population but I imagine some degree of regional overweighting is going to continue.
Of course, at state universities it’s even more so, e.g. at the University of California, around 90% of the students are from California (with some variation across campuses).