Cable shows reruns of their shows at 11 and 12 at night, networks can't do that.
They could, they just don't. I think the reason cable networks rebroadcast their prime-time shows a few hours later is because they only provide one feed to all four time zones, so what's prime time in the Eastern time zone is too early for prime time in Mountain or Pacific. An 8 PM show on the East Coast would be airing at 5 PM on the West, and people are still driving home from work then. So there are two broadcasts to hit prime time for the two halves of the country, basically. The main broadcast networks don't have to do it that way since they have separate feeds for the different time zones.