They do it this way because it cuts manufacturing cost. Its cheaper to use POP ram when ram upgradeability is not in your design constraints. Apple even has a

They do it this way because it cuts manufacturing cost. Its cheaper to use POP ram when ram upgradeability is not in your design constraints. Apple even has a patent (garbage one, describes something obvious) for second non POP slightly slower ram channel. https://www.techpowerup.com/277760/apple-patents-multi-level-hybrid-memory-subsystem https://www.techpowerup.com/277760/apple-patents-multi-level... "With two types of DRAM forming the memory system, one of which may be optimized for bandwidth and the other of which may be optimized for capacity, the goals of bandwidth increase and capacity increase may both be realized, in some embodiments," They are sitting on a solution while extracting value while market bears it.