What do you mean by handled seperatly? Both wayland compositors and X11 use libinput nowadays to get input from the hardware. But then it needs to be routed to the right application - the one that is in focus minus any global shortcuts that the compositor might want to deal with. The compositor is what understand what application has focus and thus is what knows where to send input to so it makes sense for it to handle that. It is not just about where to render windows - but manages all events such as input that applications require.