Chip stacking tends to be too expensive. Think of it this way: take the full output from a factory creating chips. Now if you stack just one chip on top of another, you have effectively cut your production in half. It also means that your chips will have to be at least twice as expensive (probably more since demand will be greater since the chips are now more scarce).

I think there is some interesting research work going into chip stacking but it wouldn't be accessible for public consumption until it's cost effective.