Why is this not part of the Apple OS?

This app saves my bacon all-day-long. And for the life of me, I can’t figure out why this isn’t in the OS already. The ability to switch between open windows - particularly windows within an app - say Safari - is paramount for quick computing. This is the FIRST app I install when I get a new computer, and you should too.

Cletus Polk about Witch, v3.9.4