Why is one "back to square one"? Pfizer vaccine schedule is the second dose at 21 days after the first dose, but as I understand, in theory, the second dose aft

Why is one "back to square one"? Pfizer vaccine schedule is the second dose at 21 days after the first dose, but as I understand, in theory, the second dose after a month, or even two months, should work equally well (except obviously reduced protection in between).