If they actually have awareness of UFOs/ET life (or even captured UFOs), I just can't see it getting revealed until it is necessary. Imagine if the US says "yeah, we've had some aliens locked up since the 50s"... everything would fall apart. Trust in government and institutions would be...
Connecting billions of people through a computer network was also essentially impossible until it wasn't. Same goes for a lot of our technology. Travel back 3000 years and tell people about a future where there will be toasters and a thing called "electricity" and they'd think you're insane.