For the chiplets, one of the other reasons was apparently they were expecting lower yields from the new 7nm process. and the smaller you can create your chips, the better the yield results are overall. (i.e. Less wasted wafer from bad silicon, when using smaller die sizes).
It also meant the IO die, which doesn't need to run at the same speed as the CPU itself, could be made on an older, mature and so cheaper node, and of course those didn't need to be made by TSMC.
All of which helps keep the costs down of course!
Biting the hand that feeds IT
