My guess is that the companies maintained the same designs even after bike shoes transitioned from thin, stretch-to-fit leather uppers to all-synthetic construction (resulting in shoes that retained their original shape forever) without changing the fundamental dimensions.

Greg LeMond raved about his discovery of Carnac shoes back in the mid-1980's. Those shoes were wide and tall across the toe box. They were unlovely to look at compared to sleek traditional shoes, though, so they didn't catch on among amateurs. "It is better to look good than to feel good."