Water isn't the only thing fabs need. They also need heaps of power, access to materials, access to equipment, access to transport, access to qualified labor, access to clients and geologically stable ground. TSMC wants most of its fabs in Taiwan because that's where it has access to nearly everything at once as most of its major clients and suppliers are on the island too.
BTW, non-salty water is actually quite rare and is getting rarer with climate change causing rain to drop in different places, quantities and times, leaving reservoir levels much lower than normal. For example, California is entering its dry season with 1/6th of its average rainwater reserve. Precipitations across most of NA over the last year were significantly below average with (severe) drought forecasts for most of the center-west areas. Water shortages aren't exclusive to Taiwan.
Many analysts predict we'll be seeing wars over access to drinkable water soon. Even VP Harris has said the next major wars will be over water instead of oil.