Forks -- as top-level named entities -- draw attention and resources away from the original project, possibly leaving both projects with too-few resources to survive afterwards.

This is why "forking" an open-source project was considered a nuclear option prior to the emergence of github.

I rarely even get pull requests from people forking my code on github, and when I do, they almost never have bothered to discuss with me prior to implementation. It's a relatively recent phenomena that is largely encouraged by the tools.