Because the language of the week changes often, and learning can be done by solving the problems of today instead of rewriting software into a version that will never be used. I mean... who still uses all the rewrites to ruby?

Even emacs was rewritten to rust ( https://github.com/remacs/remacs ), many hours were spent, and the last actual code commit was 5 years ago.... why not spend that time by making the "normal" emacs better? Or make something new in rust?