To have a more assured path of good engine cleanliness which helps reduce emission problems from oil burning and helping the timing components and turbo last longer. Volvo specs 10k intervals so he could be doing that and couldn't be blamed for it. If the book/olm says to do 10k or close to that then the vast majority of people will do so and can't be blamed for it. I'd suggest somewhere around 5k to account for cold weather. Vermont normally sees -5 to -20f in the winters. Additives deplete and the oil degrades and a gdi engine in the cold will be dumping a lot of fuel which thins the oil, eats away at the polymers, and the acidity increases. Seattle and Houston are oddly similar in winter temps which aren't cold. I and anyone else is gonna let it idle for 10-20 minutes in those sub zero temps to let the seats and steering wheel warm up and to start getting some warm air blowing instead of immediately driving away shivering because I preferred to obsess over fuel in my sump. I'll just change the oil more frequently instead.
Your two older vehicles like my two older vehicles are naturally aspirated port injected v8's that don't work that hard, don't get short tripped, and don't really dump a lot of fuel so I'm not surprised they lasted that long. Without any design issues or manufacturing defects to concern the engine it's just normal wear that will be a factor and that's something you can have control in unlike manufacturing or design. I have 340k in my 05 yukons 6.0 and it doesn't burn oil. Been using 15w-40 and change it accordingly which amounts to 7-8k.
