There is exactly one reason why they do this: So they can charge you $200 to upgrade it to 16GB and in doing so make the listed price of the device look $200 cheaper than it actually is. Or sometimes $400 if it’s a model where the base model comes with a 256GB SSD (the upgrade to 512GB, the minimum I’d ever recommend, is also $200).
The prices Apple charges for storage and RAM are plain offensive. And I say that as someone who enjoys using their stuff.