As other people have already said, most of it was fear, the other large piece was manipulation. Make them afraid of running, mutilate anyone who tries so badly that others wouldn’t even risk it. Make them think that running is pointless. You’re gonna get caught anyway, where would you go, it’s better here than “there”. Not to mention that (often) the whole town was involved in stopping escapes. If anyone saw or heard about a slave escaping then they did everything they could to help because 1. They were viewed as property, if someone saw an escaped livestock or a missing farm tool, they’d try to return it if they could, keep it if they couldn’t. 2. If a slave escaped from one farm, it was likely other slaves would hear about it and try escaping too. If someone owned slaves it was in their interest to make sure nobodys slaves escaped, because it held the image up. The more stories that got around about slaves escaping or the underground railroad, the bolder slaves might be willing to be.

That said, these were all awful ways to treat a human being. But slavery is an awful thing.