That is one of the things I'm uneasy about with shows like Stargate, despite how much I enjoy them.
Instead of just discovering the aliens on other worlds, or arriving in the present time, somehow they were here all along. Instead of humans creating and advancing civilization on their own, they were at best guided by outside influences with vast resources and technology advanced beyond what we even have today.
It also means basing your sciencey story on blatant lies.
I dislike it more when it's a part of time travel stories, where you basically have your main characters go back to take personal credit for something that real people actually did.
I never quite understood the stargate mythos. Did all the goulds live here on earth, which is why earth culture is so diverse? Ra and Apophis in Egypt, lord Yu in China, the asgard in scandinavia etc. That doesn't make sense because the system lords all control their own space right? It's hard to imagine one big happy gould family on one planet. But then how and why did earth culture come to adopt all the other gould cultures? I just don't get it. I actually really like the part about humans ultimately becoming one of the galactic superpowers at the end of SG-1 and Atlantis.
Obviously the worldbuilding and writing wasn't done under a microscope like tolkein level continuity and isn't meant to be taken seriously. It's a miracle SG-1 lasted ten seasons and they were all fantastic, even 9 and 10. Its success is mostly just good old fashioned terrific entertaining writing and chemistry between the cast and crew.
One thing Earth has going for it is that it seems to be the by far most populated planet in the galaxy. Don't know how far back in time that would've applied, but most Goa'uld controlled civilizations appear small compared to ancient kingdoms and city-states.
So even with only millions of people, that could've been enough to have multiple distinct realms without direct conflict. They could also have been separated by time. Yu and Ra were contemporaries, but Apophis might not have been. We know that all the other System Lords are younger than Yu.
Or most of them didn't stay for long, they built their empires elsewhere, with slaves from Earth.