I have had cars that required 3-4 days of routine driving under various conditions to reset the computer, but only AFTER if fixed the emissions system. What you see there is just a deterministic list of steps that are guaranteed to perform a full reset if the underlying problem is fixed
. All of them will typically be exercised by just driving around town and on the freeway for a couple of days.
If the problem persists, no list and no amount of normal driving will reset the computer. You can try until you're blue in the face, but the system is designed to not signify emissions ready status until it's convinced everything is wrong.
It sounds like the vehicle had an underlying problem that was never repaired properly. Of course it never reset, because something was still wrong. It was working as designed, your horror notwithstanding.
There's three basic states the car can be in - Good, bad, and indeterminate. A CEL indicates the bad state. You reset the codes and enter the indeterminate state, which only clears after all systems have self-diagnosed (hence the checklist above to force all systems to a known state). If the problem is fixed, it then transitions back to the good state or the bad state depending on what the computer sees. Some errors have to happen for 2 or 3 consecutive restarts to throw a code, some will throw as soon as you turn the key, and you'll never get to the good state because something is broken.
Was this a case that a code was cleared, did NOT return, but the car never made it into the ready state? There was no check engine light, it had been many days of driving, and the car still wouldn't pass emissions? That would be a PITA.