A quick Google[0] suggests that they've only started doing this heavily pretty recently, but I'm guessing new fabs in Arizona will implement state of the art water-recycling.
That said, the numbers in this article suggest that 98% recycling would drop usage to the equivalent of ~6000 homes, which still feels significant.
Low seismic activity is the benefit of Arizona that I've seen cited in the past, which I'd guess outweighs the water sourcing issues.
So 2% of that is maybe around enough water for 5k people. Arizona has 7 million people. And residential doesn't even use most of Arizona's water.
It's fine.
Surprisingly, the area is also one of the largest Taiwanese communities in the U.S., which is a bonus for the engineers and their families who will be relocated from the "mothership".
Also, it's actually quite common for high-level Intel staff to fly back and forth from Portland and Phoenix. I am not sure if the same would be said about Taiwan and Arizona, but if they have staff in SV it wouldn't be too far-fetched.
They have been building it for about a year now. Other than the possible water use, other's have stated why it is here.