Sure, it'll be more productive for everyone if I tell someone that we think they're just not smart enough to learn shit as fast as we need them to. But that hurts. So we say "You're great, but we're looking for someone with just a few more years of C++ experience". Similarly, when we think you've been acting like an arrogant dick, we say "lack of culture fit".
I once rejected someone whose English was so bad that I couldn't understand them on the phone. That's a hard thing to tell a foreigner who has the courage to call you up, by phone, in a language they probably know they're not great at, for an internship position! So I lied and I told them they had insufficient React experience. Am I proud of that? No. I'm still not sure what I should've done.
I see this trope a lot on HN that "lack of culture fit" means "wrong brand of sneakers" and it's usually nonsense. "No culture fit" means "there's something specific that we dislike about the way you behave or communicate but we don't want to hurt your feelings more than necessary".