No, you do actually gain bandwidth. By moving the memory into the same package as the processor, you can add more data lines: adding 1000 more pins to a CPU is

No, you do actually gain bandwidth. By moving the memory into the same package as the processor, you can add more data lines: adding 1000 more pins to a CPU is practically infeasible, while 1000 more signals between dies in the same package is relatively simple with the right technology. In addition, due to the shorter distance and resulting benefits to signal integrity and power consumption, internal data lines can be clocked faster than those between chips.