Several reasons, both from Apples perspective and the supplier’s.

Suppliers don’t want to purchase a bunch of equipment and scale their manufacturing capacity for a contract that could go away at any time. So when Apple shows up they are going to price that risk in to their bid or sometimes just outright say no. However, by Apple buying that equipment those risks suddenly get reduced.

Apple on the other hand has huge cash reserves and isn’t that price sensitive. Plus they want to avoid all the issues bring yet another component in house.