Because they have dedicated time. A contributor has his free time, and will contribute with work they are interested in, while still having to work at a job to be able to afford living.

But it does mean you can spend forty hours a week or thereabouts putting effort into a particular goal and maintaining the knowledge, skills and experience to keep doing it to a high standard without having to sink time and effort into something else in order to get paid enough to live on.

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.

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.

When the internet was becoming a world-changing technology, there weren't thirty years of websites to keep working and malware to protect from, web standards were far simpler, and a much higher proportion of users were enthusiasts who were excited by anything they could get and didn't mind if things were rough around the edges. Similarly, two brothers could make the world's first aircraft that flew under its own power, and yet with the combined might of everyone working for Boeing, people are worried about airliner doors falling off and an eight-day space trip has become an eight-month one. Mature technologies need a lot more effort to build and maintain than emerging ones.