Why would anyone waste enormous amount of energy making a new engine, when forked Gecko works and you can backport Mozilla's updates to it. Same way with Linux, everyone uses Linux kernel because it exists.
When/if those project die, that's when you'll start seeing replacements, and at some point the best or most popular will be seen as mainstream and will be a focus point of all the enthusiasts. Like Firefox now.
There is absolutely no reason to believe that a corporation is required to have a good opensource project, just because right now a corporation maintains this specific one.

These forks are in essence just preconfigured and hardened Firefox

That's also not true for a lot of the projects. Forks maintain their own code, porting new updates from Mozilla's stuff, but they aren't required to port everything, and that's the big reason to use them.