- Earth is special!
- Out of universe - Too help the audience connect with the stories
- In universe - For "reasons" identified by the writers
- Aliens have made Earth Special...
- BUT we have also made ourselves special!
I don't pretend to know everything about the Marvel universe, but I will make some key points I think come into play:
I think really it comes to the point of writer's making Earth special - tying in our planet to help the audience connect. This makes us all think that we matter and that our planet really has the potential for great things and that great things can happen here at any moment...
Though in universe it does seem aliens like our planet for "reasons"... most of which are coincidental and "forced/fabricated" by the writers.
- Peter's dad (Came here cause human women are attractive...)
- Thor (Came here cause he liked it... and cause human women are attractive...)
- Kree (Created the Inhumans while doing their own science stuff and stashed them on Earth)
- Vishanti (Involved with the mystical arts aspect of Earth; though I'm not sure to what extent...)
- Mephisto (Created Ghost Rider; not sure where he really comes from...)
- ETC... (I am sure there are obvious ones I am missing)
That being said, Marvel is also keyed to our genetic and scientific advancements. So Earth is also special because we are really coming into our own and putting our planet on the interstellar map.
Quickly first, (cause its "less" our fault) our genetics have started to unlock themselves creating mutants, such as the X-Men. So our own genetics are advancing making humanity special.
More importantly however, our scientific processes have advanced enough that we can create our own special stuff.Through technological knowledge we have created Black Panther, using space rocks sure, but we still did that.
But then there are the pure intellectual breakthroughs we have made such as Iron Man, Ant-Man, etc. With science we created Captain America and then we tried to replicate that with Hulk and etc. With our scientific achievements we have also opened the possibility to force our genetics to unlock, such as Deadpool. AND we have also accidentally put ourselves on the map with heroes like Anti-Man and Spiderman... go human error!
Of course, Earth is not the center of all things. The universe existed without us and our breakthroughs. Wars were waged, advancements made, heroes lived and died... but our introduction on a "galactic" stage is not purely an "accident."
True that aliens have helped put us on the map, but we would have gotten there eventually. Also, the fact that we have made it "out there" despite not being ready for it shows the potential we have and what we can accomplish. I think Iron Man going toe to toe with Thor when they first met was an important point. Despite our "tiny" frames we had advanced enough that we could hold our own with a "god."