Those forks aren't maintaining Firefox itself, just their own modifications. If a bug is found in Firefox, the LibreWolf team don't have to fix it themselves, they can wait for Mozilla to do it, and incorporate the fix once it materialises. There are forks that diverge further, but they either get quickly abandoned after their creator realises how much of a headache maintenance will be, or they're left with gaping security holes.

I'm a Librewolf fan too, but the majority of the hard work is done by Mozilla developers. Their work is very important too, but what they are doing is preconfiguring prefs, adding patches, and writing the patches sometimes. Much easier to be done as a team of volunteers.