Honestly, I think that a lot of casting directors simply cannot see beyond the idea that white skin is somehow representative of humanity as a whole. Shows like Bridgerton really undercut this idea and now newer shows are much more diverse.

I agree that it is wrong to cast only white actors in a production that could easily be racially diverse, but I find that it is mostly cluelessness that leads to this kind of limited representation. It takes courageous showrunners like Shonda Rhimes to use their influence to wake everyone else up.