I've seen actually documentaries on how scientists have found planets like earth that could have other lifeforms on it. Who says those planets might not be home a race of super intelligent beings like Humans?
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The reasons can be because their world would kind of fall apart and they would be scare.
Just look at the amount of people that are willing to say "There might some bacterial life on some other planet but that's it". As if the possibly of having super intelligent life wouldn't be just as high.
Bring up the subject in a group. And you'll find people after a while starting laugh really nervous and start to look at other people for support and eventually gang up on you just to make themselves feel better. To top it off they will say stuff like "I don't believe in little green men". Just think about how many times you already heard that. I speak a couple of language and in any language I know they say the exact same thing in the exact same way. As if it's some sort of learned behavior.
My guess is that when intelligent life is found the powers that be would wait telling us and very slowly prepare us. They may be even doing this now.
As a tip, don't look at mainstream documentaries but look at the smaller independent documentaries. There is where yo get the real information.