It's not as if we're examining an aircraft or something else which has been deliberately designed. The Neanderthal legs were apparently adequate for survival, so their structure persevered through the generations. Eventually, nature's blundering system of mutation produced a better model -- our own wonderful selves -- and the days of the Neanderthal were numbered.
Our own "design" isn't exactly optimal. Lots of humans suffer from bad backs, dental problems, poor vision, allergies, etc. Very few of us are models of athletic perfection. But our species seems adequate for survival, so here we are. In due course, our technology may permit us to "improve" on our design. That should be interesting.