It goes back 25-30 years when some big company car users and leasing firms put pressure on certain manufacturers to extend the service intervals. At the time most big company car operators had strict limits of car had to be disposed of when it reached 50,000 or 60,000 miles.
So any long term damage and excess wear was never to their account, as got rid of it before it happened, so manufacturers to be competitive and get these bulk orders, all pushed up the service interval when competitors did it.
Then second users started to get problems with oil starved turbos etc at higher mileages, so most offered an alternative schedule for those not cruising long distances (basically allowing for lot of cold starts, and engines not warming through. But this has led to complete muddle where people with mixed driving don't know if they should be following the long variable, or the fixed interval servicing.
The synthetic oil does last longer, but problem is if drive lot of short journeys in winter, engine barely warms up, and a fair proportion of oil stays as thick heavy cold stuff in the sump. You don't need much imagination to work out if only part of the oil is regularly circulating, that bit will deteriorate quicker. That's why after a decent journey in colder weather the car often feels more lively because got the oil warmer and better circulated.
A friend of mine with a company car took some oil (just prior to an extended oil change) from his dipstick and dripped it on a microscope slide, then did same on another slide after the oil change. Let's just say since then he wouldn't extend the intervals on his own car having seen the difference. He described it as comparing a jar of clear honey to a jar of treacle with bits in bit