OIL - Change or not to Change

I last changed the oil on my 71 Duster 318V in Oct, 2017. I changed it today, it had less than 1000 miles sine the last change
Everyone I talk to tells me the oil will break down over time and needs to be changed.
The oil is sitting in the oil pan, not exposed to light or air for that matter.
If the oil will break down in the car's oil pan, then why does it not break down in the plastic bottles the auto store has on the shelves.
I imagine some of the bottles of oil they have may be much older than what was sitting in my oil pan.

Yes, I know what the rules for changing the oil are, but this seems kind of ridiculous with exactly 989 miles since the last change

I can't really say either way, I think considering you are in AZ which is pretty dry it will be OK but 1 1/2 years is kind of a long time... I'm really curious what an oil analysis would pick up, hit up Blackstone Labs for a test kit and send some of that "old" oil in then you'd really know how much junk got in there from sitting for that long.

Unless you engine is completely sealed (which it shouldn't be) the oil is still exposed to air, just less than if it was sitting out in an open container.