Their reason was that applications want to control everything about how they look, so they don't want compositor-provided decorations that clash with the rest of the application window. Whether that's a *good* reason... Well lots of electrons have been used up in that discussion already.
https://gitlab.gnome.org/GNOME/mutter/-/issues/217
https://gitlab.gnome.org/GNOME/mutter/-/issues/1143
... and probably many other places.