Because I still believe in the ideals of responsive web design. If a desktop user wants to use my site in a tiny window (maybe to tile it next to another window

Because I still believe in the ideals of responsive web design. If a desktop user wants to use my site in a tiny window (maybe to tile it next to another window), they should still be able to use keyboard shortcuts. And if a user with a large screen wants to use touch interaction (maybe they have a Surface Studio), they should be able to. There's no reason to make multiple sites each with their own flaws, when you can just make one site that can do anything.