As others have said it is a huge amount of work to maintain a fork of such a complicated piece of software.

Especially around security: web browsers constantly process potentially-malicious data, which gives them a large attack surface. Every browser regularly has new vulnerabilities discovered which must be fixed. Hard forking a browser means that, even ignoring any bugs in the new code the fork has added, every time a bug is discovered and fixed in the code they forked from someone needs to analyze the upstream's fix and port it to the fork. The more they diverge, the more work this is. Failing to do this work lets any malicious website exploit the bugs and install malware on users' computers.