That's mostly because people are thinking about other things. We understand that and anticipate it. If my computer doesn't understand me, it has no excuse as it can't distract itself. It isn't going to hear me next time by "concentrating harder" like a human can. It's going to keep failing.
I have a different experience - many people speak with a mumble or a mushmouth and no amount of concentration helps me disentangle it until I can get them to speak more clearly.
Sure, but if you repeat your utterance there's a good chance that the conditions will have changed the second time around- maybe background noise will have subsided or you'll have swallowed that bit you were chewing on and so on. It makes sense to ask you to repeat a couple of times even if it's a computer you're talking to.
