Earlier quoted context omitted.
'Emirates A380 Shower Spa': https://www.emirates.com/english/experience/our-fleet/a380/ First class passengers get 30 minutes, including 5 minutes of running water. You can see videos of it on YouTube.
Don't planes burn enough fossil fuels without carting around a few extra tons of water quite unnecessarily. Market efficiency, yeah right.
Second, the water they use per person weighs less than the weight of the extra passengers that could be fitted in if the arrangement wasn’t first class cabins but was instead economy. First class passengers are where all of the margin is.
Finally, you don’t seem to understand what “market efficiency” means because it has nothing to do with this.