Its because Lex has a point. Superman is a god, and like all gods there is the risk that people will just choose to rely on him for everything. Thats why the "inspire the masses" part of Superman is so important, and why he tells common cops "The park is safe because WE'RE here, not because I'm here."
Because Superman cannot be the savior of the world, people have to save themselves, strive for greatness, even suffer, know loss, and rise above it.
Superman represents the very real danger of a safe but weak people coddled by a superhuman savior, a Messiah that stands in the way of mankind developing.
That is why there is always such a big emphasis on Superman being an icon and a symbol to people more then simply the guy who punches away the world's problems. Because the world cannot survive its problems being punched away, and Lex knows this. Most of the stories where Superman goes insane are the ones where he cannot accept that he cannot use his powers to save everyone, he has to let humanity deal with its own problems to some degree or another.