What is the appeal of the bath tub to a cat?

My 5 month old DSH spends several minutes, two or three times per day, playing in my bath tub. He’ll be running around playing, then heads to the bathroom, jumps in, rolls around on the floor, paws at the drain and the fixtures, then moves on to something else.

He has never had a bath, though I have cleaned him using moist wipes made for cats. He has also never attempted this while the shower was in use. So I can’t think of any reason he would have any emotions tied to the tub, positive or negative.

He does not seem to be hurting himself or anything, just getting a little moist from the residual water from the prior shower, and he doesn’t seem to care. I’m fairly sure there is minimal chance he could ingest any soap residue from the tub floor, and even if he did it would be trace amounts and very dilute. (No more than I have accidentally rinsed into my mouth on rare occasions, and I survived.) So I haven’t bothered trying to prevent this.

I guess I should note that he has many toys, and multiple sources of fresh drinking water, and uses them all.

Mostly I just want to know- why does he do it?