That's just lazy of the chromers. I worked at a chrome platers when I was young and we chromed everything including wheels. Never was there a need to split rim from centre. I think its just so they can get their polishing mops into every tight space.
There's no other logical reason to separate rim from centre, in fact I think its dangerous as you would then be welding a chrome surface to another chrome surface. Far better to do it in one piece and retain the integrity of the wheel.