It's kinda stupid how they say
"Superman is wayy too powerful"
but yet when they compare him to say Thor, Hulk and Juggernaut.. they go on saying
"Those three would stomp Superman easily"
and they NEVER complain about those three
That's the sort of illogic
some Marvel fans use when decrying DC heroes, such as Superman.
For instance, I know of some who hate Superman (because he's an alien), and Green Lantern (because he got his ring from an alien), because, of course, "that's impossible", on the other hand - in their opinion - of course it's perfectly possible
for a radioactive spider bite to create Spider-Man, or a gamma bomb blast to turn a man into the Hulk!
Nothing against the Hulk or Spider-Man (or their fans), but I've always thought it is straw-splitting to maintain that some super-powered heroes are more realistic than others because of the use of very arbitrary and selective criteria in passing judgment in what is, after all, an extremely implausible and unrealistic medium; superhero comics.
Getting back on topic, the answer to your question, "Why do they always want Superman to be "weaker"?", well the main one is that it is a lot easier to write Superman that way, rather than as a god-like being, as Denny O'Neill -for one - observed years ago.