They're fucking mods. WARNING: INCOMING ANGRY CHOST (yes that's also a Reboot reference.)
Games and DLCs made with profit incentive in mind often fucking suck
Modders spend years making mods for shits, giggles, their own desires, and probably internet points. The most probable reason why they take so long is that they have to go to work to stay alive.
Modding, just like any other artistic endeavor, would benefit immensely from people not being forced to work.