Given the current circumstances/laws/top secret classifications, whoever does "show you a body," what you're really asking them to do is potentially spend the rest of their life behind bars, if not be outright murdered in the process, for your satisfaction.
Outside of making that sacrifice, these people are doing everything they can, up against the keepers of the greatest secret humanity has ever known, who currently operate in cold blood with a complete lack of oversight or consequences.
Now, would the person who definitively proved to the world that we're not alone likely ultimately be pardoned? Probably so. Would you be willing to take that risk/bet, though?
Point is, it's easy for you to demand proof when you don't have to consider the consequences of what revealing that proof would mean for actual human beings with lives and families. Still, hopefully, someday someone is brave enough to make that sacrifice, and if they do they'll be an American hero.