Why doesn't Arizona have solar panels on every rooftop?

I know it would be expensive for old houses to get them but what if they were built on all new houses . I mean people have the option of adding a 20,000 - 30,000 dollar pool in why not make solar panels mandatory. I also know that for the panels to work they need to hit the sun for most of day well make sure the roofs of new houses get lots of sun. All new shopping centers should be required to get them also. Ive also seen covered parking in az with solar panels on top of them they looked really cook. I live in Az