Mountain Park sits in the same cold-moderate climate band as the rest of the region, which means real freeze-thaw cycles each winter. On a door with original steel panels and the factory weatherstrip, that cycling works moisture into seams and joints that were never sealed for 45 years of service. Wood trim around the opening swells and shrinks with it, which is often why an old door starts binding against the jamb on one side only.
The other driver is simply the age of the spring. A torsion spring is rated in cycles, not years, and a door installed when these houses were built has likely gone through several spring replacements already - or is overdue for one now, especially if nobody's kept track. Combine a tired spring with a track that's lost its factory alignment over four decades and you get the uneven travel and opener strain that's common on houses from this era.
We don't have a breakdown of exactly how many Mountain Park doors are original versus replaced, so we won't guess at a number. What we can say is that houses built around 1979 are old enough that original hardware is the more likely finding, not the exception.