It's just far more likely that they don't fill any of the presidents in on that stuff. A lot of people seem to think you become president and suddenly all secrets are open to you and you get the keys to area 51. I guess I can see why people assume that, but it's far more likely, if not an outright certainty, that no president is told. It's a position that is only temporary, often with leaders with little to no military experience and pretty much nothing to be gained by telling them.

Just consider that there is no way in hell that Trump was told anything and he actually kept it secret. At the very least he'd have tried to take some kind of credit when those ufo vids were released by the pentagon. Just to big a risk to tell someone that is only gonna be around for 4-8 years and then instantly becomes a liability concern (or liability certainty in Trumps case)