If nothing else, because people who screw up should have better manners. You step on my foot, I notice and ask you to get off, and you immediately do and say "

If nothing else, because people who screw up should have better manners. You step on my foot, I notice and ask you to get off, and you immediately do and say "sorry" -> I forgive, forget, move on. You step on my foot, I notice and ask you to get off and you delay, express surprise, finally do, and then explain, at length, that you didn't notice, aren't that heavy, meant nothing by it, but you're sorry if I'm offended (and it's the third time it's happened this year) -> kind of a big deal, because I'm getting very tired of this cp.