Well, from reading her books, I think it may be her ruling passion: something she has to do, something she takes joy in -- of course it takes effort, but the effort itself may be rewarding, and when the idea for a new world or species comes into her mind, she's probably eager to explore it.
There are some of her worlds I would like to revisit -- I keep hoping, I keep hoping -- and I do feel there are possibilities going to waste, but if she's following her bliss I don't think you could call any of it a wasted effort.
Besides, sometimes when it seems that she's moved on for good, she does go back to a previous setting. I thought the Morgaine books had ended as a trilogy, and years later she wrote a fourth book, and ended it in such a way that there was plenty of scope for more.