Because it costs money to store and inventory small parts. It needs a sku and a price for the part, then a sku/price for the repair/replacement. How many small rubber feet should an Apple Store keep in stock for the occasional customer who comes in and loses one? Easier to just replace a whole part that contains many of the smaller attached parts.
I’m pretty sure all the MacBook feet are exactly the same, certainly the MBP line are. They could store 10000 rubber feet in a pretty reasonably sized box....
I was amazed when two of the feet on my 2014 MBP came off on the same week.
