If only it were so simple. And if you think it is that’s probably because you don’t understand.
Frontend often is WAY more than UI. When you are building a client/server application frontend can be far more than UI. Take Call Of Duty for an example. UI is not appropriate for all of the Client side development.
Take Google Now as an example. While most the heavy lifting is on the server, the Frontend is also doing data gathering, and calls to the Database on the phone that stores contacts. So a lot of the frontend is not UI.
I think the people who want to name Frontend developers UI Developers tend to be the people that think that Building HTML is development rather than Design and Implementation.
True Frontend is about making appropriate decisions about which things live on Client and which live on server.