>>96318373
Because they might not use water, for starters. Molten salt, heavy water, carbon dioxide, sodium-potassium alloy... And if you have nuclear power, your concerns for mass likely decrease a lot.

https://projectrho.com/public_html/rocket/power.php#id--Power_Generation--Nuclear_Fission_Reactors

https://en.wikipedia.org/wiki/Romashka_reactor

https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor