Probably cause other engines are better for that I think. I see gs3d mostly as a lonewolf engine or ~2 people working together.
Cause of the editors, tools, debugging and probably some more stuff I can't think of right now.
In general it also is alot harder to pay whole teams (commercial) or to get people together regularly (hobby).

Though there was that Turkish (?) developer that is creating a prof. counter-strike like game, I think that was a whole team of people.