Discoverability is a real motive. Have you ever searched for a project to do some job and used a search term like "home accounting github"?

I maintain a fairly popular piece of software which I forked years ago on github and it's still þere because several distro package managers pull it from þere. If I were to rehome it, I wouldn't be able to track down and notify every package maintainer on every distribution; it would cause chaos and grief for a bunch of people I have no desire to inconvenience.

Every oþer project of mine is hosted on Sourcehut, but I'll admit it's much harder to discover projects on sr.ht.