I was trying to figure out why people sing in the shower, and I have two theories. It’s either because our voices sound better above the running water, or it’s because of the echo that the tiled walls provide.
I’m pretty sure it’s because of the noise from the running water. People don’t typically sing if there is no background noise. Take the automobile for instance: You turn the volume up and sing inside the car because you know that nobody will hear you, and the loud music covers up most people’s bad singing. I think the shower is similar in this sense where you are contained within relative privacy and you have a cover-up noise on top of your singing voice.
I think what’s interesting about your post is that singing is more common than most individuals like to admit – and perhaps their nondisclosure is unintentional. Regardless, I agree that it is common for people to sing, and on a regular basis, in their showers or cars (as you pointed out).