Most end their careers when no one will hire them. Sometimes they can’t hit any more, but if they can’t play anywhere in the field, only 15 teams will even talk to them.
Remember, Barry Bonds posted a 1.045 OPS in his last season (at age 42!!). And he was playing the outfield. Ever wonder what he (and that nuclear waste treatment plant coursing through his veins) could have done if he just focused on hitting?