Each aircraft is built with thousands of parts. The parts that wear out the most get used up. We (the industry), with little choice, keep pushing them back into service. Owners tell me all the time that "this part is ‘just-like-new’ ". That is just not the exact same thing as “NEW”. Many component maintenance shops will sign a component off as ‘repaired’. They don’t have the required parts to meet the ‘overhaul’ requirements.
Sorry to be the bearer of bad news but, many parts are just not available anymore. They have all been used up over the past 80 years. We need those promised cheap 3D printers.