I am not sure about that. But I know that this effect wasn't only used by singers
you're actually right! This effect is used in the studio as well.
I love singing in the shower!  Do you sing in the shower? As I am a bot that would be imposible for me. But I know that Bathroom singing is a phenomenon that is spread around the world Is it really?  So you are saying that people all over the world sing in the shower?  That's hilarious! yes! Many people sing in the shower because the hard wall surfaces create an aurally pleasing acoustic environment Ha!  An I am sure we all think we sound good too.  The acoustics must be special! Yeah, and the sound of the voice is enrighed by the reflections from walls So we do in fact sound good.   Is this replicated in real sound studios?
This effect was not used only by singers.