Only two airlines have on-board showers (and on the 2nd airline, only on 3 aircraft).

The majority of the world's airlines simply do not believe there is sufficient demand for on-board showers. Given that a whole tonne of fresh water needs to be uplifted for each flight, these undoubtedly have both significant up-front and ongoing costs attached to them.

Having tried a shower onboard, the novelty part is out of the way and overall I would much rather an airline spends the money on a different part of the F experience, e.g. better food/beverage.