I suppose languages evolve around the way their corresponding population brains work. People can still learn other languages, or be native to other languages, but there is a language way that is the best fit to some people which is related to biology.

However, natural languages evolve naturally, which means that they don't just suddenly randomly change, and that change is very gradual. So things tend to get stuck in historically-motivated local maximums that can be very different for different languages because of their different histories.

There are some plausible theories around biologically motivated language features, but this tends to be about the environment - e.g. some sounds seem to be more common in languages spoken in high-altitude areas.