If alien races have interbred with human kind since long time ago, then there is no need to look for them outside us.

Wilier races had interbred with human race during immemorial.
Therefore no need to look for them on anywhere but in ourselves.

It is written, 'seek, and ye shall find'. But first, 'imagine what you seek'. Otherwise, you will end up searching everything everywhere forever.

If you want to find wilier race by common sense,
then you have just narrowed your searching area.

Let us think for a moment about alien, regarding possibility of the existence of other civilization beyond our dear planet earth. We probably conclude that it's all just hoax, or some rumors issued by ruling government for some purpose. Now, we can contemplate a bit about it by asking this simple rhetorical question, 'How could possibly people be able to think about something unknown, which have never been popped out in human minds at all?'

Parents restrict their children accessing weird sites on the internet. Same reason, the ruler of this world restricts public accessing the sites of alien civilization everywhere.