Marvel theory: In the MCU, humans are an experiment.
I believe that humans might be an experiment in the MCU. Here is why:
First, consider the following: Humans are not aware of aliens, but all the other races we see in the MCU are. Also, humans developed different languages, while it seems that all the other ones speak the same language, AS IF THEY DEVELOPED TOGETHER. Experiments are normally in a predictable, controlled environment.
Next, think about how all humans were able to become so powerful. Doctor Strange with his magic, Iron Man with his suit, Captain America with his shield and super-soldier serum, all the Inhumans, Graviton(Literally the person fated to destroy worlds). This is much different from all the other planets.
Third, think about how compatible humans and infinity stones are. Whether Carol Danvers with the space stone, Wanda with the mind stone, doctor strange with the time stone, and yes, Jane Foster with the reality stone. Also, with peter quill, I actually think the weak part of him was the eternal part, and that’s why he wasn’t as compatible.
Fourth, power is attracted to earth, whether Thor or the infinity stones.
So, my theory is, some beings(Likely very powerful, like Eternals or infinity stones) created the human race and put them on a perfect planet, away from everyone else. The human race was highly powerful and compatible with the infinity stones. Nobody really came to earth, because it was strange and far away. Eventually, Thanos broke the experiment in the avengers. The purpose of the experiment was to have a race that was powerful enough to handle the infinity stones and powerful enough to defend the universe. However, now that the infinity stones are gone, the true power of the humans is gone, and so the Eternals are stepping in to help.