Every company has a frontend team, and all of those frontend folks want to get promoted. Building an in-house component library is a straightforward card to play.
It's basically the natural outcome of reasonable people responding in reasonable ways to seemingly-reasonable incentive structures.
(I do agree there's probably a more efficient way, but I think it might also be one of those slippery-slope situations where there's a lot of gravity pulling sufficiently-large teams to eventually have their own component library. In other words: Suppose they start off with the best of intentions and repurpose an existing library. Then come the inevitable months of customization, forking, racing to integrate upstream updates, etc. Finally, someone pipes up and says, "Hey, why don't we build our own so we can control our own destiny?" Stakeholders get excited, and then it's off to the races.)
