The critical angle of attack seems to be at all (most?) airfoils around 15-20°. Why is that? Why is it in this range and not lower or higher? Is it just the result of optimizing airfoils? Or is it some inherent property of the air an airfoil is moving through that determines it? (Let's limit the question to subsonic flight.)
PS: I read the answer to "Does airspeed affect the critical angle of attack on an airfoil?" and don't think that this is a duplicate since the other one was more generally about "what are the factors that determine the critical AoA" while I'm asking why the critical AoA seems to be always in this range for most airfoils on planes flying nowadays.