New motor oil's detergents and dispersants don't have a lot of contaminants to attach to, so they interfere and compete more with the antiwear additives. The new antiwear additives gradually chemically transform into more effective antiwear species, due to heat and rubbing. Other reactions create various esters and carboxylate acids that provide antiwear abilities. These are the things that make used oil improve as the oil ages.
There are negative things happening as well as oil ages. Abrasive particles accumulate in the oil. Fuel and water dilution increases rapidly early on, then stabilizes, as long as outside temperature and usage stays the same. If there is a coolant leak into the oil, that is obviously bad. Deposit precursors increase with usage. For a while, that is not a problem. Eventually, the oil will start to allow deposits to accumulate. It may still be providing excellent antiwear protection. If it has too many abrasives, it will allow increased wear to occur, compared to itself when it was younger. Many oils that "fail", do so by being used too long and they leave a deposit mess behind. They do not fail due to ineffective antiwear additives.
