“…they are extinct.” says an article just published in Astronomy.com.
Here’s another plausible explanation.
We have ourselves evolved to such an attitude that we leave the natives of the Amazonian forest alone – unless we covet their land, of course. We no longer impose our (technological) civilization on everybody who is unable to resist us – like we did to the Incas, for example.
How about the ‘surviving aliens’ keeping mum while waiting for us to grow up some more?
The next step being for us to stop killing each-other…