Actually quite simple John, though because it may not happened on YOUR car does not mean it does not happen on others.
Here are things that happen to doors: they sag. They sag because hinges wear. The door closing and locking mechanism can wear. The mounted parts, can, in a few decades time, loosen up a bit, changing the alignment geometry. Anything that moves, wears. Tighten thing us out of line, and that exacerbates wear.
So, factor in all the things that move on a door. Add a few things like someone, somewhere along the line, leaning on a door too much. Or, body flexing from improper lifting. There is no shortage of ways to slightly alter the geometry of door fit. Be thankful it has not happened to your car, but it certainly can and does happen to others.