> What the hell do you need a library for?

Think of a grid of cards, and all the possible drag, hover, and drop state transitions it can be in with any other card, and the animations required to make it look nice

Now also consider that you need to persist the final state on the backend, and handle things like rejection, cancellation, failed updates/errors.

It becomes a massive state chart and web of complexity

I mean, maybe we're missing context here. Is that what your boss was actually asking for? 99% of web applications (or, hell, desktop applications) don't have ultra-interactive, fully-responsive-animation drag-and-drop. If he was asking for functional drag-and-drop and the team was implementing top 1% cosmetic drag-and-drop, that's certainly one avenue for a mismatch in expectations.