Is probably one of the most offensive and insulting phrases people can say. It's often used to say "This exists why do you need to make that", but then bicycle wheels exist, why do you need train wheels? It also implies a wasted effort and an insult towards the person making the effort, as there's work to get to the same stage, and it kind of implies that the problem has been solved, which normally highlights how they are either bitter against people solving problems, or have incomplete knowledge about problems... But the response is simple.
Oh, why are you making a machine gun? You're just reinventing the wheel. We already have smoothbore muskets what more do we need?
Somehow, this phrase is deeply engrained in the unconscious of software engineers about libraries and the way things are. Or, the person saying it doesn't understand the domain well enough to understand the improvement- or is so blind to be unable to perceive it. Blind leading the blind!
"Why would you reinvent the wheel" as bitter resentment against learning how the wheel works or belief in forgoing the process to a conclusion by the silver platter you received it upon.