That presupposes that every engine on here is either:
- easy on oil to begin with
- ran through easy intervals (i.e. minimal idling, thorough drive, no short trips, etc.)
- used in a strictly passenger capacity
- stock and not modified
Many of these could just simply be put under the "normal" vs "severe" interval.
In my opinion, when asked the "why not go longer" question. It is simply this... and I think many on here would share the same sentiments -- I had the opportunity to service the vehicle and I was at an appropriate interval that of which met my particular "minimum" mileage or duration for that required service.
While I don't ever plan to have things go awry and life get crazy. You never know. I like to be comfortable with my service intervals where they can potentially go the long haul if need be. If something came up and I had to make an unexpected trip to family, do I want to be stopping at Uncle Ron's Hot Dog garage and tire center for an oil change... no. Nor do I want to think that I need to service the truck before I leave on an impromptu trip.
To sum it up, for me, its about being prepared.