If you create a library that uses async, you're forcing everybody that uses the library into async as well (with the same executor).

If somebody writes a library now that's generally useful but uses async, it forces others to use async or rewrite the library themselves.

On the one hand a lot of people put this down as whining about free code, which is somewhat true, but the infectious nature makes the whole ecosystem less useful if you want to build something non-async.