It's not really that simple. R_R_X is the expert in so he'll probably weight in, but the sculpt is just part of it. Once they cast it, they need to paint it, yes. But they need to make all of the bodies, all of the clothes, all of the accessories. Packaging the sheer amount of stuff that comes with this figure takes time, as well as quality control/inspection. Plus, you're at the mercy of how quickly the factory can work based on the volume of orders that they get.
Plus, in this hobby, having extra time going into production is definitely a good thing. It usually means they're committed to getting everything right and not rushing things to get it out the door.