And massive return of artifacts (as opposed to one or two once every decade) will open some very inconvenient flood gates, like the fact that Britain was not the only empire in the history. And some "non-western" countries were or are empires too. Imagine that after India get their stuff back, Afghanistan comes next to India and asks for their share. That would be very bad optics. While without actually receiving stolen or gifted stuff, countries can keep their moral high ground and common external enemy (which is conveniently very far away).