It’s a strategy. They are in a position that they don’t have to meet demand, not going to hurt sales one bit. But by making a small batch upfront, they can monitor sales and manufacture accordingly. Keeps cost down as they are not sitting on extra stock ever. It’s also a way they can add exclusivity to a product that is manufactured by the millions.
I'll add to this. I think Apple partly deliberately constrain the manufacturing. Why? If they had huge supply to begin with, the sales would fly through the roof and then very quickly fall again. This is not what Apple wants to report during the few quarterly reports after the yearly iPhone release. If the release is constrained, they can drag out the huge iPhone numbers across multiple quarters, otherwise for months of every year would look like revenue flatlining, shareholders would be angry. Look at the Airpods, still 6 weeks waiting on the website. Just the 'look' of huge demand and whatnot draws people in and some are probably now ordering them just cos they have to get their hands on this new in demand product. Psychology marketing 101s.