I'm not sure how many companies know this, but your best UI/UX people are very likely to be your developers. Why? One very simple reason: you can't be that good at UI/UX unless you know front-end development well.
UI/UX designers that only use mockup tools are really only guessing at the final UI/UX. They cannot run the app and see issues and make improvements.
Developers don't have to guess at the UI/UX because they can make changes to the real app and try it again.
If a UI/UX designer gives a developer a mockup, then the developer will solve a bunch of UI/UX issues that the designer isn't aware of.