While their is no real reason why the Ferengi could not have been known to the US Air Force since 1947 there are a few points you should consider:
(i) In 'Encounter at Farpoint' TNG, the Enterprise seem surprised that the Ferengi are interested in Farpoint station, almost as if they've never heard of them before! Similarly, in 'The Battle' TNG, we learn that Picard and the Stargazer fought the Ferengi at the Battle of Zeta Maxia, but that at the time they did not realise the ship was Ferengi, and again, it almost seems as if no one had fought them prior to that! The Maurauder class ships are fairly distinctive, I'm certain an Ensign could have identified one!
(ii) In 'The Last Outpost' TNG, at Picard's request, Data divulges all the known information about the Ferengi to the bridge crew. There doesn't appear to be a great deal about them, considering they've been known for 417 years. (But then we don't know much about the Andorians either.) The major problem here is that it says they were a complete mystery to the Federation until this first contact was made.
(iii) In 'Little Green Men' DS9, Nog tells Captain Wainwright that a fleet of three hundred Maurauder-class attack cruisers orbit Earth and will soon drop their cloaks to begin transporting Klingon shock troops to the surface. Wainwright appears convinced this is the truth, so if he made records of the Ferengi, why didn't he also record the Klingons? In 'Broken Bow' ENT, the Generals have never heard of Klingots before! I think the records must have got destroyed during the Third World War.
(iv) The Ferengi are not members of the Federation ('False Profits' VGR.) Why didn't the Ferengi Alliance join the Federation, with all its possibilities for trade. Maybe we will find that out yet?
