Metalurgy, lubricants, steel sleeves, lack of maint. I've seen a few old engines years ago where the rings would clog up and pump oil like crazy, not really worn out. Think a lot of folks pulled the T-stats and ran the engines too cold. As mentioned, carbon under a valve. Remember one old mechanic in town, around 1950 would drive the car, flog the engine real hard, rev it up a couple grand and slowly pour water thru the carb to clean carbon out. If this failed, he would overhaul it.
ray