In terms of evolutionary biology, it is easy. Weaknesses are normally part of the process, and the environment ruthlessly screens out the weak. If you don't let that happen, you will have a larger number of weak individuals.
Kind of like glasses. I wear them, but a few hundred years i could perhaps be a jeweler. Maybe.
Instead, I make a good living.
Thank goodness we as a society are too civilized to engage in such ruthlessness toward the weak. I'm glad we don't "let that happen," as you put it.
I would not have survived long in a primitive society with my bad eyesight and slenderness. Either famine or a large animal I wouldn't have seen till it was too late would have got me.
In terms of evolution a 50,000 or 100,000 years is a blink of an eye. The change cannot be explained by that.