Right, business logic and UI are distinct creative tasks. While one may enjoy both not everyone does, also requirements based design for business logic is a different mental processes than user story based flows for UI. One is very concrete and the other is known to be pretty mushy until the user actually touches it and sees how it responds. There are developers who do not like to iterate through fuzzy details and prefer very concrete data contracts. So they are very happy to stay on the other side of the REST contract. The good part is once they get a glimpse of the new world, they realize how much better it is for all parties involved. It's one of the few evolutions I have seen in the tech world where the grass truly is greener, on the other side, for all parties.
"(...) how happy they where not not have to deal with UI requirements and be able to focus on the business logic."
I've noticed the same, but I still don't understand why is that.
I'm curious why so many backend developers are not happy with working on the frontend part?