You've gotten the technical they're coded to attack a player that hovers their crosshair over their head or torso from up to 64 blocks away answer far too many times, so here's a few theories of mine, ranging from serious to outright jokes.

• They're shy.
• They dislike eye contact for whatever reason. Maybe they're autistic? (That'd explain their problem with water too. Sensory issues!)
• They're insecure. If you were a ten-foot-tall abomination with no features aside from glowing eyeballs, you probably wouldn't want to be looked at either.
• In an inversion of our expectation to make eye contact with someone, looking someone in the eyes is considered rude in enderman culture. Their language is backwards English after all, maybe other aspects of their culture are backwards.
• Steve is just really, really ugly, and they don't want to look at him.