A man walks into a flying school and books some lessons, lots actually and is prescriptive about what he wants to learn: no take offs, no landings, just a bit of turning and “straight and level”.
A flying instructor thinks it strange and tells the authorities.
Another man buys a one way ticket and pays in cash for a transatlantic flight.
A father finds it strange and tells the authorities. The Authorities don’t seem to tell each other.
So lots of people employed to keep us safe and to detect terrorists fail to notice either of the men who tried and perpetrated attacks on airliners. It beggars belief.