aelf
Ashen One
Because we need to have faith that our government/military/the man can protect us from outside dangers, and they wouldn't be able to do that if there was a super-advanced alien intelligence flying around, probing people's anuses.
People would freak out. Things would change.
Unlikely. There are so many dangers from which you have little to no protection as it is. How many people nowadays are scared of some big bad external enemy? Unless there are some remarkable experiences, such as in the case of terrorists, not many people will be genuinely spooked.
If alien encounters are significant and frequent enough, then maybe there would be more reason to think that they would cause a significant change. However, that doesn't seem like the case, otherwise how do they keep covering it up? And if highly advanced aliens really do want to interact with human beings significantly, I doubt they can be stopped, so trying to shield the public is also useless. When they come, they come. Maybe it's better that we know they might.