Maybe it's because we're not as evolved as we think. If aliens did appear, how many kooks would want to attack them because they are different?
If they land in one country, would other countries get jealous or scared, thinking that country now has an advantage? Shit, what happens if the neighbouring country had nukes; would they launch?
More likely is that if aliens had detected life here, Earth would be marked as a red, no-go zone, because of how unstable, tribal and violent we are.