The angels could send a person back as far as they wanted. All we were told was 'back in time' - this 'only about 100 years' thing is not from the programmes. Just because 2 people were sent back not that far it doesn't mean that's the limit of their power. It's your own supposition. So there's nothing from the shows to say that they couldn't send people back further - before civilisation, for example. They can also send people to different places - in Blink, the girl was sent from London to Hull.
Anyway, as people have said, they do that to feed on the energy, which they didn't need after the Byzantium had crashed. They just needed to get people out of the way.