Once you see it you can indeed not imagine how you couldn't. Some people see it the first time, but it's a small amount of people. This video just demonstrates how humans can only focus at one thing at a time, and when we're multitasking, we're actually doing little parts of different tasks one at a time but very quickly after each other, kind of like a single CPU core. And if we tightly focus our attention to one point, we are not aware of other things that might be relatively close to that point.

That is also how magicians work, drawing your attention to one particular thing, hiding the secret of the trick from you, sometimes even in plain sight, like in the video.

Or pickpockets, who might bump into you and picking your pocket at the same time, where your attention is focussed on the sudden impact, keeping your attention away from your walled being taken.