The brain doesn't always see everything, but the thing with experiments like that is precisely that it's not applicable. You do that once, miss it, someone tells you and you're surprised you missed it. The second time you watch it you pick it all up. But if you train for literally years, maybe decades then it's not being asked to do something out of the ordinary and not being used to it.
That is the thing, these are professional referee's, we're not talking about the guys first day on the job, we're talking about probably 20 years after officiating their first game. You get better at basically anything you do the longer you do it.
If you are someone you can't be vigilant and spot the small things then you aren't suited to be a referee. The simple fact that someone gets to the level of prem league referee over sunday league should already weed out all the people who aren't good at keeping a good eye on what is going on around them. Likewise his main job is to watch what is going on effectively with the ball and in the immediate area around it. So his focus should have been directly on the ball and he absolutely should have seen the hand ball.
If he'd missed someone say near the goal keeper pulling a shirt or kicking someone that is expected if not just understandable but missing something precisely where his focus is supposed to be isn't understandable. His entire job, his entire career is supposed to be based around his ability to detect such things.
The video even explains and says, if you weren't expecting other things to happen then half still saw the gorilla, when you expect the gorilla you see it but maybe miss other things. A veteran referee should after probably 20 years in the game be expecting to see everything and not miss most of these things.
I already said though, mistakes are expected and technology has been the answer for 15+ years, it got even easier to implement with the advent of wireless devices, tablets. It would now be incredibly easy to get a tablet held by the 4th official in which the ref can run over and have a look.
Until your average fan is demanding it, it won't happen. It will fix almost all severe unfair decisions almost instantly in football. With the majority of key game changing and potentially season changing decisions being made correctly we suddenly have a fairer game, with less cheating and more focus on football.
EDIT:- it's worth pointing out that if you sacked enough ref's and the quality dropped even lower then video technology would become a much more urgent thing. You can't not fire people who aren't good at a job because other people might be worse. The converse is because these guys aren't being fired, other people aren't getting the chance to step up AND people are training to match the level of the current no where near good enough referee's. If their standard is too low but we accept it as fine, then frankly that is the level we are asking every future referee to achieve. Firing some would potentially make these people take notice and do something about it. Better training, more referee's, and helping them be better referee's by any method. The best way to help them make the right decisions is video replays, accepting ref's that aren't remotely good enough is just maintaining the status quo.