SPOILER: Running a Country
A superhero is like a military unit with one brain and one body. Some are the equivalent of a platoon of infantry. Some are the equivalent of a tank division with full air support. There are three things that keep the military from taking over in any nation where they have not in fact taken over.
The first is indoctrination. Stable nations go to a lot of trouble to teach everyone that military force should always be subordinate to civilian authority and possibly some kind of constitution which has authority over the civilian authorities. American comic books and their foreign imitators instill in children the idea that while superhuman powers may entitle you to bend the law in the service of "justice", they _never_ entitle you to make the law.
The second is a lack of unity. Military leaders have to be confident that they can personally swing enough of a following to pull off a coup before they'll try. If your super population consists of just a few uber-powerful individuals, Kryptonians, or something, then it is fairly easy to organise a takeover if you can just persuade a few friends to go along with it. If you have just one Superman (or Von Doom), then it becomes child's play. If on the other hand you have a fairly large population of individuals of more moderate powers so that they can't entirely shrug off the threat of conventional firepower then it becomes far more difficult to organise a takeover and the risk isn't so tempting.
The third of course is a lack of inclination, separate from indoctrination. Regardless of whether you think it is wrong to take over, actually taking over the administrative duties of government is a time-consuming pain in the ass, and most heroes (and villains) have absolutely no abilities that would make them actually more capable and successful at running a government. How many people would want to spend hours a day shuffling paper without any clear idea as to whether they are accomplishing anything when instead they could be out flying around trashing "bad guys" and rescuing people and modestly accepting their thanks?
Consider Sailor Moon. In her future history she really does "take over the world". She becomes the queen of Earth (stepping into the power vaccuum left after a massive catastrophe that kills much of the planet's population). Now I've seen a lot of Americans react negatively to that idea. After all "we had a revolution to get rid of foreign monarchs yadda-yadda". Sometimes they write unfanfiction about it where she's portrayed as a tyrant, implausible though it might be that someone chiefly characterised by profound empathy for everyone she meets and a lack of inclination to do any mundane work, would make a likely tyrant. But consider that Naoko Takeuchi was born and grew up in a monarchy where the monarch in question has historically played little or no role in the actual administration of their supposed realm. For the last thousand years or so, Japan has had a grand total of one emperor who actually ruled. For the rest it has been shogunates, and military juntas, and for all of Naoko Takeuchi's life, a parliament. Although NT doesn't go into details about how the future works, I would expect that while Neo-Queen Serenity may her face on the all the money, in peace-time (which is most of the time) she no more rules the planet than Queen Elizabeth rules Canada
So, maybe the heros do take over for a while because the normal governments have done something stupid like trying to launch an anti-mutant pogrom or trying to declare World War III and launch the nukes. But if they do, they'll soon find that actually running a large government is a frustrating, and time consuming business even if the public is entirely behind you and one that they haven't actually been educated to do and aren't all that interested in. What's more, they are so personal powerful that they have little to fear in the way of consequences from giving up governmental power. So the natural thing to do is to hand over the actual responsibility to people who are interested, educated and experienced. For Western heros (and military men) that naturally tends to mean holding elections and turning over actual authority to the winners.
