Otto, I beleive it is a combination of casting and machining techniques (cylinders and pistons) and piston ring technology changes over the years. It's probably a lot more complex than that but because of emission regulations engines need to be performing like they are broken in from basically new. Those changes are also why modern engines are often seen to exceed 200,000 miles without rebuild when back in the day 100,000 miles was considered a milestone.
I once had a discussion with a person who worked for US EPA about the agency's desire for vehicles to be in emission compliance when they went to th scrap yard. It was not a position that I could wrap my head around.
