The problem is, when Gene Roddenberry sent to Central Casting for non-human actors, they sent Lassie, Mr Ed and Flipper. A case of working with what you have. All the Humanoids because, all the Humans playing them. The make-up budget was tiny and CGI far in the future.
The Precursors was putting a lampshade on it. The good thing about writing you own stuff is the special effect budget is unlimited.
I know

My point was since completely human-looking aliens outnumber even the ones that are merely humanoid (Trill, Cardassians, Klingons etc.) and are found in every corner of the galaxy, it should stand to reason that it's the "default" outcome of the "guided evolution" the Precursors created, so the Precursors should be human-looking ones (meaning no prosthetics) instead of rubber faces.