With my engines it's an accumulation of little things.They have good compression but I know the bores have a lot of mileage on them.My present ride has new Valve stem guides and stem seals as well which are usual suspects but the also the crank,although within spec,is 2nd hand with new shells on.
All these little things can add-up.Some people will own bikes that have not been abused and have kept well will most likely use very little oil.Old bikes are old bikes however there are even new bikes from some makers that use excessive oil.If you do a Google for motorcycle excessive oil use you'll find some interesting things