I think it’s something to do with the lack of external stimuli, the sudden alone time with nothing to focus on but the thoughts bumping around in your head. It’s a great place to make plans and come up with solutions and ideas.
Given how many of us solve the problems of (our own) universe in the shower, I believe I’m yet to read in any novel a contemplative shower scene. I wonder why that is? Not that I’m planning one myself but it was just a thought I had … when I was in the shower.
Perhaps it has something with having to have the character DO something while they are there, you have to describe something other than the thoughts in their head….