Showers are generally considered to be relaxing separately from the “shower thoughts” phenomenon.

Couldn’t the relaxation be a factor in generating shower thoughts?

I suspect that essentially none of our non-ancestors were predated in a hot spring, unlike walking etc, so there may be an environmental cue driven induced relaxation that doesn’t exist for many other activities.

Yea, you relax, and then your brain produces random thoughts about things.

I suspect it's just about getting the space to relax, which is why I frequently have thoughts when staring at the wall, or taking a walk, or washing dishes, or doing any other myriad activities which are relatively easy on brain processing.

Solitude is extremely powerful.