Lots of major free software projects are hosted by the project creators to avoid being screwed over by proprietary services.
Examples
If Microsoft were to release a competing link aggregator; then they could persuade GitHub (Microsoft’s subsidiary) into locking or removing the main repository for Lemmy’s source code.
or a self-hosted gitea instance maybe ?
We already have both of those, self hosted gitea and codeberg.
That’s an extra cost though