That's called the "Zoo Hypothesis" which basically states that the aliens wherever they are in space can see us but we can't see them. They don't want to mingle with us because they don't want to tamper with the natural order of things.
They’ve kept their distance not because we’re imperfect, but because of our right to pursue our own destiny. Diversity is something that everyone in the cosmos is assumed to value, so life-bearing worlds should be left to their own evolutionary development.
It's certainly an interesting hypothesis especially with all the fiction illustrating abductions and experimentation by these aliens or world domination in some respects. So it is plausible that aliens are keeping their hands off in order to let nature have its course.
But you would think then that we are imputing arbitrarily characteristics to these entities. Who's to say that they are an advanced and benevolent extraterrestrial civilization who would rather treat us animals in a zoo, needing protection from the rest of the universe?
It still stands that if there are aliens, they just might think it equally reasonable to invade our planet considering we haven't found anything remotely close to Earth's environment in outer space. It's a question for those who study the stars.
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