Well, let's consider that an open manhole isn't something that we come across everyday while walking down a sidewalk. We take it for granted that a manhole cover wouldn't be off. We assume that a sidewalk is a safe place to walk, which it should be.
There are many dangers that we encounter everyday that we are conditioned to keep an eye out for. (Like cars in a road, others walking on a sidewalk, dogs in a yard, joggers, kids playing on the side of the road or birds flying above that may crap on your head.) An open manhole cover is just not something that we deal with everyday. And if you do see an open manhole cover it's normally sectioned off with some sort of plastic border. There was a crew working in a manhole by where I live for weeks. ATT was installing fiber optic cable in my neighborhood and there was always a partition around the manhole. It would be impossible to walk into that hole if you were texting or with your eyes closed.
One thing that I always try to do when understanding ANY situation is to put myself in the shoes of the party in question. Imagine yourself as that person and doing what that person was doing. It takes a good amount of "outside the box" thinking and creativity, but try it. You might realize that your view point might change. You might realize that your original thinking was correct. Anyway, it's a good habit to get into in any situation. Try thinking from the brain of someone else in any situation. It makes you a smarter, more well rounded person.