I can attest to the fact that, having worked with hundreds of potential F-1 students, it is not uncommon at all for Embassy officials (globally) to deny visas without providing any excuse for denial.
Sometimes it seems like the denial is simply "I/m having a bad day, and do not want to deal with this"., or "I do not like you" On occasion the applicant has simply been told the official does not believe the applicant has enough motivation to return home at the conclusion of studies and is a high risk for flight.
On another occasion,(resolved favorably by a phone call from a sitting US Senator, serving as Chair of the Senate Committee on Homeland Security), the Embassy official originally denied issuance of the visa based upon HIS opinion that the course of study was a waste of time and not in the applicants best interest]. After the Senator challenged that official's professional expertise in the subject matter and professional career counseling, suddenly the visa was granted. Ahhhmazing!