I think for a lot of games there are a couple of different issues with that. First, most adult games don't bring in enough money to support paying other people to help with development. So most of them are solo projects. Second, if you hire other people to do some of the work then you have to spend a decent chunk of time making sure that whoever you hired is actually producing something that matches what you want.
Personally, I would rather handle most aspects of game development myself. I think the 1 exception I might make if a game I was working on became successful enough to support it would be to hire someone good at animations to turn some of my still shots into animations. I'm not good at animations. I'm sure with practice I'll get better, but that's the one area I've found where I compare what other devs have done in their game to what I can do and admit to myself that I would not be able to do an animation that good myself (and I mean an original one, not just a premade posing series like quite a few games use).