I think owner neglect has a lot to do with why some of these motors consume. Also, think of the people who run K&N filters. I doubt they go into high mileage without oil consumption.
Valve seals, piston rings, oil leaks, would be the top three reasons for oil loss. There are other reasons, but those three make up a lion's share. One other thing to keep in mind, most engines will consume more oil as the ambient temperature increases.
Irrespective of what the manual says, I've always considered 1quart in 1500 miles the benchmark for a "problem". Now, after having said that, I have seen severl vehicles which have had a "boatload" of miles on them run just fine and consume a quart every 1,000 miles just like clockwork.