Economy

But it's a dry heat!

Seriously, low humidity sounds great for manufacturing fine components. And you can raise the facility temperature much higher for human comfort if it's dry.

Water cooling sounds like a maybe problem?

Big cheap plots of land.

Phoenix is cheap compared to LA or something, but not that cheap. It is geologically stable though, has relatively consistent weather, easy access to cheap nuclear power, and has a preexisting semiconductor industry.

You also need a fuckload of water and the energy to purify it all.