They weren't removed because they are part of the official street names. This is evidenced by the fact that non-LED street signs at all City of Phoenix intersections (i.e., ones that aren't at stoplights) include both the prefix and suffix. This is also the case for most all street signage at intersections without traffic lights in Glendale, Peoria, Mesa, Gilbert, Tempe, Scottsdale, and Chandler, among other cities. Even if a city in the metro decided for aesthetic or other reasons to not have the prefix in their signage, the streets are all part of the same or similar grid systems, and the official name used by locals would still include the prefix.