I remember 10k service intervals outside north America 15 to 20 years ago and oil was not synthetic. The main reason for the shorter intervals here now is the mindset of the customer I suspect, they don't believe in maintaining their vehicle strictly to the schedule and often run past the recommended interval , so manufacturers have short service intervals to compensate for customer stupidity, illustrated for their fondness for cheap quick lube service. When I moved to Canada 12 years ago I could not believe new vehicles still had 3K mile oil change intervals and people expected a full service while they sat and had a coffee like a tire change. Why a 5k interval, why not 3k interval or even a 1K interval? Because it is diminishing returns, the quality of the oil will no longer be a factor in how long the vehicle lasts, with even the newest best quality oil an internal combustion engine operates on the basis of friction and will stil be subject to wear. The 1K mile first change was eliminated a long time ago and car engines last longer than ever, go figure.