For a large part of their history, superheroes have been reactive rather than proactive. That is, something bad happens and they react to it. (There are exceptions—early Superman stories where he's fighting against social ills might be an example, but stuff like the Authority is definitely an exception.) As a person creating adventures, that cuts out a huge swath of possibilities that you get in other games. Hyperman doesn't set out to become President, or if he does, it's in his secret identity. (Uh, there's an adventure right there: a certain politician is running for election for a powerful office; the PCs have evidence that he is secretly a superhuman. What do they do? To make it more interesting, assume that the politician/super has powers that will make it possible for him to destroy the possibility of replacement, such as mind control. Go.) This is also at the root of things like Reed Richards Is Useless, where the wonderful toys he creates don't have an effect on society.
Comic books as an entertainment and story-telling medium have a couple of reasons for this, some of which I've seen before. You can't make the world too different, because you want to feed into the central conceit, that this is a power fantasy for our world. Stories where X happens and the hero fixes X and sets things right have a long, long history in all genres (you can consider Gilgamesh as an example) and it's essentially the structure followed in Westerns and adventure fiction.
I'm not well read on this topic, but the reason that I haven't seen elsewhere is this: having the characters act proactively for a goal makes them less trustworthy because they have great powers.
I don't know if that follows for you, but here: in fiction, our hero is always attacked in his weak spot. He is always metaphorically outgunned. Hamlet can do many things (we're told he's an excellent swordsman, for instance) but what he can't do is make up his mind about a philosophical problem, and that's exactly what he gets: Is the information from the ghost of his father trustworthy? I have no doubt that if he believed the ghost fully, he'd kill Claudius, end of play. But he needs confirmation, and puts into effect the plans that eventually kill people.
There are other stories, but the idea of struggling, of trying and failing, is usually in all of them. It's hard to struggle if you can obliterate the enemy with your eye-beams.
But there's also this, and maybe this is my Liberal White Guy Guilt and Bias talking: The greater the power that a character has, the more you have to make them dependable and trustworthy if you want them as heroes. And I wonder if reacting to the situation isn't a way of confirming to the audience that this character will only pick up the weapon when it is necessary. The Western hero does not use his superior gunplay to take over the town, and the President does not use his ability as Commander-In-Chief to roll over the small nation. In the same way, the guy who can lift a tank and bounce missiles off his chest does not take over Area 51, just to see what's there.
Am I saying that you can't do a story about a proactive guy who happens to have super powers? No, I'm not saying that at all. But I am saying that you have to be as careful about reader empathy as you can. It's like, uh, the screenwriting book, Save The Cat: you can get a lot of audience buy-in by having the hero do something good in the beginning, and not using your powers for personal gain is perceived as good.