If you're buying a rider or moto, chances are it shouldn't hit below the belt line if it's an honest take on a jacket that was originally made for a specific purpose.
Fashion jackets: 5 zips/bombers/etc etc etc. Fit differently because they weren't conceived for being worn on the bike.
While you are generally correct, there are notable exceptions. Brimaco, from Montreal, was famous for a longer jacket that went considerably under the belt line. the zipper , though, usually ended above the hem.
Old riding "shirts" also were longer than cafe racers, which have been around awhile, (obviously), but really become the jackets of choice of "recreational" motorcyclists, as part of a really brilliant marketing move, iirc, by Suzuki, to differentiate "sport" riders from 1% and motorcycle gangs, in the early 60s.