I've received in-stock items from MACS in a few days and one back ordered item, 14-months after I ordered it !! I've also had similar experiences, both bad & good, from Bob Drakes.
The problem, as I see it, is that the Vendors order a quantity from their Suppliers and try to figure out how many they will need for the year. If they run out early, they have to order more parts. The foreign Suppliers may make them wait their production turn for 6-months or more, so everybody down the line has to wait. That's the price we pay for available, affordable parts for a relatively small market. They might sell a hundred set of mirrors, for example, a year, which is small potatoes to the manufacturer.
Another problem is the tax laws in many states that penalize companies from holding inventory.
Bottom line, thank heavens we have these Suppliers for 50, 60, 80-year old cars and be patient in waiting for the parts they make available for us or make them yourself, if you can.
Or take up stamp collecting....................