I think the theory is that disabilities rarely occur without warning.
If you get chest pain while driving a bus, you can pull over to the
side of the road. If it happens in airplane, it may be a while before
you can find a safe place to park.

I've seen statistics on medical-related aviation accidents, and the
number of these that could have been prevented by a more rigorous
medical is close to zero. Compared to CFIT, VFR into IMC, and
stall/spin accidents, the medical requirements don't come close to
justifying their cost and inconvenience.