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You only have to look at this planet to see how hard it is to leave the atmosphere. How many species have ever lived on earth? How many of those would be considered intelligent? How many of them have left the atmosphere?
Stop thinking so hard. I mean don't you think alien civilizations have laws and such? They can't get funding to send their people to earth. They get sued too much so they stop testing their long range spacecraft. Worst of all, they get dumber over time; as technology increases they evolve to be more simple.
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