Cosmoline said:
We tend to assume it's because we "surprised" them. But that doesn't jibe with their incredible senses.
I've only ever had one bear encounter, and it was anything but an emergency, so I'm far from an expert on them, and I agree that humans are often to blame for not being aware enough of their surroundings...
But with all due respect, your comments make the animal nose sound like some kind of active sonar system, that detects anything within a 360* radius for miles. That's just not biologically possible. No animals nose, no matter how sensitive, can detect you unless your scent particles are delivered to its nose, either by contact (directly smelling an object) or by the wind (sniffing the air).
Unless you are up wind, then there is no nose which can smell you. Its not possible.
I have a bit more experience stalking deer, whose noses are only 40 times better than a humans, and still better than a dog. I can tell you 100% that I have surprised them before, especially practicing in the off season when there guards are lower. Even as an amateur, I have belly-crawled within 20 yards of a grazing whitetail. I could have gotten closer, but after a few hours of inching along with the gusts of the breeze I got bored, stood up, and walked away. I have never seen a deer so surprised.
An animals sense of hearing works in a similar way. Even if it is super sensitive, there brains will block out noises that are loud or repetitive - like a babbling of a loud rushing river. In this way it is possible to mask the sound of approaching hikers. The bear may indeed have physically heard the sound of their walking, but not registered said sounds as anything different than the splashing water it had been ignoring all day.
And then you have the fact that its simply impossible for any animal to be 100% alert, 100% of the time. They have lapses in their awareness just like people.