The math doesn’t work that way, given the fact that Harvard and other top elites turn away many more well-qualified applicants than they admit.
If, hypothetically, there is a 1:10 chance that a well-qualified applicant to Harvard (based on stats alone) will be admitted — assuming that the other half the applicant pool who get rejected simply aren’t qualified — then that would mean that absent a preference, 9 out of 10 well-qualified legacy applicants would be rejected. So if, hypothetically, they want legacies to have a 1:3 chance – they do need a preference to achieve that rate.