People like the actual text editing environment and either don't care about the extra functionality an IDE would provide or just add such functionality as plugins. Since text editing speed isn't the primary limiting factor in producing software you may realistically not be much or even any faster whichever environment you choose unless the environment is actively horrible.
Fair enough. I would never argue that Vim's text editing functionality wasn't superior. However, the advocates I've encountered all touted it as superior, "...in every way." and I just haven't seen that.
