This fails to take into account that some people actually prefer a mid or lower tier UC, but apply to Cal or UCLA anyway for whatever their personal reasons are. Automatically assuming that someone would drop everything and go to Cal or UCLA over another UC is awfully presumptuous, and the admissions staff know that. Even if the UCs did coordinate acceptances (which I’m fairly certain they don’t), you can’t automatically assume which UC someone would choose if they were accepted to multiple, because it depends on a lot of different factors unique to each person.