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.
I bet haha, I guess anyone can sing in the shower, I even heard that kids to it too lol.
and depth."
This habit was reported (with an attempt of explanations) centuries ago. For example, Ibn Khaldun in Chapter 1 of his "Muqaddimah" writes:
The bathroom singer is an ironic reference to mediocre or amateur singers.
Jon Anderson had tiles installed in the studio, to simulate the echo effect of one's vocals in a bathroom.
This effect was not used only by singers. Paul Simon wrote:
"Weird Al" Yankovic in fact in 1979 recorded his first single,
Bathroom singing
Bathroom singing, also known as singing in the bathroom, singing in the bath, or singing in the shower, is a widespread phenomenon. 
Many people sing in the bathroom because the hard wall surfaces, often tiles or wooden panels, and lack of soft furnishings, create an aurally pleasing acoustic environment. The multiple reflections from walls enrich the sound of one's voice. Small dimensions and hard surfaces of a typical bathroom produce various kinds of standing waves, reverberation and echoes, giving the voice "fullness