Comment by rbanffy

A lot of it is just two layers - the chip and the interconnect below. If you have a chip that generates a lot of heat, you can stack another one that generates less heat on top of the hot one and use it as a heat spreader (a lot of mobile chips are like that). Also memories have only a small part of them being used at any given time, so they are a good fit for tall stacks (current flash memory does that too)