Why not try? Because the cognitive dissonance when people's eyes and brains run into a large variety web-based and app-based screen fonts. Web based font performance is still terrible. So.. because: the brain doesn't like it.

Different presentation fonts in web pages, desktops, etc. can start to have the experiential impact of (gui) skins, or non-standard dotfiles.

Please note the things I'm leaving out of this... print, books, static things, etc.