We still don't really know what happened with JFK. Or Roswell. It took years for info on Charles Manson being a CIA experiment to come around.going to have to second Skall here, people are pretty bad at keeping secrets and something this big....yeah not likely.
The more likely thing is that we've been working on incredibly high tech secret shit for awhile which considering the size of americas black budget and the amazing drips and drabs that have come out is much more likely.
Keep the circle tight, and keep strong incentives for keeping the secret, and yes secrets like this can be kept for a long time.
Shit, this incident itself is direct evidence of that. We know from evidence that has leaked that this stuff is real. Doesn't mean it's aliens, but they've managed to keep the secret for decades.
Besides, your argument boils down to "I don't believe it because I don't believe they can keep secrets," when the only reason we know about this stuff now is because on multiple occasions, they have failed to keep the secrets.
Something is going on. That much is true. Aliens? I'm not claiming that yet.