It's not. Acceptance rates don't take into account the quality of applicants. Based on a large and reliable sample of top college applicants I've seen, Harvard, Yale, and Princeton are still slightly more selective than Stanford when controlling for the quality of applicants.
Is it easier to get into Stanford than Harvard?
Class of 2025 Acceptance rate
Is Stanford harder than Harvard to get into?
if you're worried about admissions, they're both difficult to get into, but Stanford is actually more selective than Harvard is (by a small margin).