But in the gifs provided, the fighters DON'T just close their eyes, they pull their heads back and away from the punch, which is a good defensive reaction.
Not to mention that if you are talking from a biological point of view, then it does make sense to close your eyes when an object is flying toward your face. Closing your eyes protects a vulnerable, soft part of your body from damage as well as tenses the rest of your muscles in preparation for impact.
Aldo literally just closes his eyes and moves his head back slightly, then tries to play it off like he didn't just flinch by shrugging and laughing. In NO WAY would he have prevented Conor from landing that left hook that he feinted by doing that. If Conor wanted to hit him, he would've done it right then and there.
Conor jumps back like 6 feet away from Nate even though there was no way Nate was going to be able to hit him from that distance. He was already at a safe distance, that step back was completely unnecessary.
Neither time did those reactions actually protect those 2 from a potential attack, all they did was show that they were scared.
What you are saying about biologically could be legit, but the whole point of martial arts is to go against our natural human instincts which want us to shell up and close our eyes when strikes are flying towards us.
When you are practicing boxing, MT, karate, or any striking art you are never supposed to close your eyes when strikes are coming at you. You are either supposed to slip, parry, block, then either get out of the way or counter strike. You are supposed to directly fight off that instinct that makes us want to turn away/close our eyes to a strike. That's what training is for.
You are supposed to defend yourself at all times.