Which is why they cleverly moved the central story of a software change from the commit to the PR. Commits migrate automatically with the repo. But migrating PR descriptions and comments takes engineering effort.

Rhetorically, why can't folks add the feature of PRs to Git? Git is fundamentally just a graph manipulation engine and data structure. It would just be another object type, in a layer which sits above the normal commit graph. Obviously there is no "just make X" in software, but this feels reasonably achievable, and would have tremendous benefit for the cost.