There are many planets across the universe, each with variable abilities. Most aliens on world are refugees of exiles from their planet, trying to hide in plane sight and appear human (either by already looking human, being able to turn human looking, or using a technological disguise). Forced to assimilate, many aliens lead normal lives, with many hiding their ancestry from those they're close to, only those who take on a heroic persona are able to show off their alien heritage to the public. Aliens on Earth are a minority, making up only about 0.1% of the world's population, the majority who arrive choose this planet due to it's pre-Bridge Drive technology making their more advanced knowledge beneficial, as well as the fact that it is under the protection of the Sentinels. But, of that 0.1%, there is an even smaller percentage who arrive here on accident, fleeing their planet in a random direction, and just ending up on Earth.
This has been going on for decades, maybe even centuries, who knows what famous historical figures who are thought to be human were actually alien. In the time that aliens have populated Earth, their more advanced technology has been scattered across the world, things that were maybe mystical could have actually been alien. Some modern and historical Powered used this otherworldly technology to become the heroes we know today.