Same reason some of us think it would be cool to meet an intelligent alien race that arrives at our planet, we don't like to deal with believing other more powerful beings would fuck us over, but rather be nice to us and give us all their stuff.
Superman is the ideal of a perfect hero, he's indestructible and morally righteous, of course that comes with an issue as he's nice to us for no particular reason, the only real reason he should be nice to us is because if we all died out and the planet went cold and barren he'd have nobody to talk to, I guess he could enslave us and furrow up some concubines or something, but he might get pissy if he thinks people don't really like him.
Humans don't like to believe that our species are imperfect, we spend too much time comparing ourselves to the animals that we know that are dumber than us, even when they're particularly smart like dolphins, we still zoo them up and watch them for entertainment. Even with Superman being objectively, infinitely more incredible than anything evolution has spurred up here in the ridiculous amount of time this planet has existed, we still found a way to bullshit him into being indebted and mindlessly kind to us all. I may be wrong, but as far as I know he and his race have a very similar mentality to humans, and no human that decided to do the shit Superman does would last very long without punching some whiny cunt through a wall.
He's nice because we make ourselves feel better by thinking that if a being(s) like him existed, he/they would put up with our shit out of the kindness of their golden alien hearts. I'm sure I've heard Superman be compared to Jesus before, which probably functions in kind of a similar way.