As a WPF developer, I was confused when I discovered that I'm not considered front-end because my platform isn't web, even though I clearly deal with user interaction and the front end of an application.
One thought I had was that desktop applications don't have a separation between the front and back ends (the UI and domain, respectively) like the web does. However many applications do have this distinction, especially in the enterprise. Most of the desktop applications I've developed professionally were merely desktop clients for data served up and received by web APIs. In this sense, the client is very much the front end.
In this answer, the writer states that "front-end" must run in a browser whereas "client-side" could potentially include desktop applications.
So… why does "front-end" only pertain to web development?
Best Answer
The premise of your question is wrong. When you say that you "discovered that WPF is not considered 'Front End' development", what you really meant is that you discovered that large swaths of the industry are so web-centric that they can use "Front End" as shorthand for browser stuff. This is by no means a universally accepted definition for the phrase, as Wikipedia makes evident.
It's all about context. In the corporate/enterprise dev world, WPF is still very much a "front-end" skill. Even the author of the post you linked to said only "front-end...generally runs in a web browser based interface" (emphasis added).