That's ecosystem inertia: in the days of yore Terminal was pretty damn terrible, it only got tabs in 10.10 (Sierra) and xterm-256 in Mountain Lion (10.8), so people got into the habit of recommending iTerm 2 and then kept doing that even as Terminal became usable (though not necessarily great).

And iTerm still has feature edges e.g. truecolor, or better multiplexer support (Terminal only supports vertical pane splitting).