Four decades of wear
That gap matters more than it sounds. A door and opener installed in the late '70s has had over forty years to work its springs, stretch its cables, and wear its rollers down to the shape of a worn tire. If your door is original to the house, it's very likely still running the same torsion or extension spring setup it shipped with, and that's the part most likely to fail without warning - usually as a loud bang followed by a door that won't budge.