If every US President knows about aliens, and yet any past President hasn't told us, then there's must be a reason for that.
Presidents are not dictators, they may simply not be allowed to, or else.
The most likely reasons for this is because the goverment doesnt have much if any data on aliens, They might have some ufo sightings/strange reports from airforce personnel but probably nothing as concrete as what you might see on tv about area 51 etc. The uk goverment gave access to their alien/ufo documents some years ago and although there were some interesting reads it was all mostly boring sightings that could be anything and in this day and age when everyone and their dog has multirotors then the sightings are only going to go up.
Guaranteed 99% of all reports that the goverment have classified is because its jets or aircraft that are top secret but nothing that would blow your socks off.