Because in my point of view that is called a UI developer which is a different area. It exists on web/desktop/mobile. A Front-end developer is not necessarily developing a UI - even that you call any website a UI -, but it differs about it's purpose. When building a UI for desktop apps the goal is to give the users a tool in their hand to complete certain actions. However websites can have several purpose like presentation, media delivery, news, etc.
The separation of front-end/back-end I think it's coming from the fact that there is a server side and a client side. This is how it works and there is no exception on the field of web development. On desktop side you usually use one technology, like .net in which you may already have the tools to build the full working application. For the web, front-end connects together different technologies (Node, PHP, .net, Go, Py, etc) using the same language/language stack (HTML, CSS, JS).
Front-end became a really advanced and rapidly evolving territory in the last couple of years. It's really getting hard to have up-to-date knowledge. That's why I feel that the separation is legit, and that's why I think being Full-stack developer is a lie nowadays, but that is another topic :)