California has a metric fuckton of water issues. Not exaggerating. If it weren't for active irrigation efforts via water taken from other states, the majority of California would be desert.

Then there's the active water mismanagement due to farming that's been going on for over a century that's resulted in some amusing fuckups.

Water tables aren't inexhaustible. They need to be refilled, and alot of that comes through... rainfall. If you're actively storing and containing said rainfall, multiplied over an entire state, that can have some further devastating consequences down the line. Hence, illegal.

Gotta grow all those avocado for soyboys morning toast, afterall.