Even natural history museums and anatomy textbooks often draw quadrupeds in impossible gaits, but cavemen did pretty well.
In my experience, good reproduction of life requires turning off the noise of what you think you see, and drawing what you actually see, (or have seen). I would guess that more modern artists spent less time observing four-legged creatures, and also have more noise covering their internal representation of one.
I remember a somewhat-related experiment with urban Australian kids and Aboriginal kids. If I remember correctly, they were given a board with either several stones on it, or several geometric shapes on it. The shapes wre then removed, and the children had to recreate the positioning of the shapes or stones on the board. Urban kids performed much better with shapes, and the Aboriginal kids performed much better with stones.