What was the rationale for not requiring plates? California is also the only state I've driven in where the freeways don't have mile markers. How are people supposed to report the location of an accident or breakdown?
California has some mile posts and some numbered exits, but you're expected to report location by named exit. Ex: North on the 405, right after Pico Boulevard. Etc.
The California highway system was extensive before federal highway standards were developed, and California was exempted from exit number requirements. In ~2000, California decided to add exit numbers 'over the next ten years' as signs are replaced; twenty years later, it's still not complete, but Google still likes to give directions with exit numbers only, as if that were useful.