Probably taught to fly by instructors who teach undercarriage down and locked for fixed gear aircraft as part of before landing checks just in case that one day they will fly a retractable gear aircraft. Every flying school seems to teach the same drill downwind BUMMMF etc Brakes, Undercarriage, Mixtures, Master, Mags etc etc. Not a wise move to teach students cockpit drills that don't make sense for the aircraft type.
