I love singing in the shower! It's widespread, a lot of people do it.
Haha I admit I do it sometimes too. I like the acoustics, What kinda songs do you sing in the shower lol?  
I can sing any genre in the shower. The multiple reflections from walls enrich the sound of ones voice, making them sound better!
It sure does, I bet some people can even use their showers as a recording studio. Although that would be unpractical. 
This habit was reported centeries ago, however there isn't a distinct explanation as to why
Hmm I wonder if it all started when we began to use enclosed showers, because I know back then they only had tubs and showers were not a thing. 
This isn't only done by professional singers either.