Why do my eyelashes, meant to protect my eyes, fall into my eyes? Why do my cheeks/tongue sometimes get in the way of my teeth so that I bite them? And why do they then get inflamed so that I continually byte them for the next few days?
We are all a bunch of biological goop resulting from random processes. Don't expect optimal solutions from evolution. There is no "why".
There is a why and reasons we find and insights we glean. The denial of a why is itself an anthropocentric take on biology, influenced by Darwinian thought. But any biological system we study we need a why, otherwise the corollary of your statement essentially leads to no learning or understanding, because everything becomes “arbitrary” and explained away by randomness.
Ironic to say it’s not optimal, really we don’t have the full knowledge. Often when we learn more we learn how little we really know about biology.
We are all a bunch of biological goop resulting from random processes. Don't expect optimal solutions from evolution. There is no "why".