The problem with focus-follows-mouse on macOS is the top menu bar.
If you had it, and tried to mouse to the menu, the menu would switch out from under you.
I have some notes in a file somewhere about how a Mac-native focus-follows-mouse could even work, because I want it. It would end up being a rather different implementation than the X Windows style.
I've been searching for a sloppy-mouse-focus implementation for OSX for _years_. Pretty sure there's something fundamentally incompatible in how app windows are managed.
Example. Mousing over another window on the way to the menubar.
https://github.com/sbmpost/AutoRaise