Lawyer by day. Rules lawyer by night.
I think people also have this preconceived notion of Superman based on really hazy memories of the Christopher Reeve Superman movies - which were pretty bad.Eh, I feel like that's way more specific than the complaint is usually made.
At a certain level I think people react viscerally to the ending and end up painting with a broad brush. I mean, there are few movies I've seen the internet have such a knee-jerk reaction to as Man of Steel. It's a flawed movie, but I find most of the time people really go beyond what they need to trying to make a point.
And yes, they were bad.
With the exception of some very good scenes, the movie itself is mediocre. For every "You've got me?! Who's got you?!" from Lois, you get idiocy like "How many Rs in massacre?" You've got Superman telling a reporter about his weaknesses. You've got a frankly stupid plot to flip a real estate investment in Nevada? The comic relief is, by and large, dad jokes ("Bad vibrations?"). Then there's the cringey fact that Superman has one POC with any lines in it - a "jive-talking" Black stereotype.
Superman II is pretty bad. The progenitor of Marvel bloodless destruction in scenes like the fight in downtown Metropolis coupled with clumsy writing. Not to mention that not only does Superman straight up murder Zod in the end, he's happy to do so. Go back and watch it. Superman depowers Zod and then happily drops him into a bottomless pit. And then Lois murders Ursa. After doing all this, Superman then goes back to a diner to get revenge on someone. That's the Christopher Reeve Superman that people claim to prefer - a murdery Superman who beats the crap out of humans.
Superman III is a live-action cartoon. Not to mention that it's dated and riddled with plot holes.
And Superman IV: The Quest For Peace... well, it's Superman IV: The Quest For Peace. It's only "redeeming" point is that it finally addresses the fact that Superman has the power to do more than rescue kittens from trees and can actually make a lasting difference.
Superman Returns tries to retcon away III and IV and succeeds in making Superman into a mopey stalker.
From what I can tell, when people say that they don't like Man of Steel or Henry Cavill's Superman, what they're really saying is that they prefer silly and stupid Superman movies.