Because they're meant to be 4:3, and some idiot behind the scenes doesn't realise that. American Dad has the same problem.
The most recent series of Family guy was actaully 16:9. All 4:3 episodes are transmitted as 4:3 and the aspect ratio is switched within the first 3 seconds. What experience do you actually have of directing television presentation, only before you call people idiots it might be nice to get informed yourself!