It's not entirely true. Otherwise we should require anyone to use Gnome/Gtk because there are apps for KDE/Qt that don't look the same.Each application looking different? Stupid idea.
You don't need the same all the time, unless you're some old grumpy fuck or so. In society you don't need only white people. Nowhere do you need everything
to be the same, i.e. "consistent". Sometimes when the "consistent" sucks or doesn't suit your needs properly you want non-consistent.
So the idea that apps looking different must be a stupid idea - is a stupid idea.