One of the creators of Fallout, Tim Cain, periodically releases blogs on various topics. Recently, he decided to explain why he is not creating new franchises.
I love developing games, so it is really unclear to people why I don't make new franchises [IP]. Other people have made much more money from the IPs I created than I ever received myself. And not "a little," but much more... People who just work in a team with franchises I made many years ago earn more money than I received for creating those IPs.
Tim Cain admitted that there might be a way to solve the financial issue, but "he is not good at it" because he is simply a "good artist."
And overall, the creator is satisfied with his work:
If you are really interested in what I want — I am still making video games, and I love them, and they make me happy. I am tired of enriching [other] people through my own creations. Can you give me a reason, other than very selfish ones, like you "want something new," or "other people want to make even more money" — why I shouldn't just continue creating what I like?
The developer would like to see a fairer distribution of profits in the gaming industry; he drew an analogy with painting: "When a painting is sold, everyone makes money."