Phoenix to LA is always a bit of a problem for cyclists. There is no 'good' route.' Just a rideable route. Much of it on or along I-10 in AZ. CA is more strict than AZ about cyclists on their interstates, and it looks like Google knows this. Your best bet may be to just let Google maps do the routing, in bicycle mode, adjusting when common sense suggest.

You are normally required to exit an interstate in urban areas, and never allowed on inner city expressways.