It’s a physics issue. Signals in modern devices are extremely high speed; the easiest and cheapest way to combat this is to bring components closer together. For example, the wireless radios, RAM, and processor in all modern phones exist as one chip. They essentially put the CPU and wireless magic on the same silicon die (or on a separate die in the same package) and pop a RAM chip on top (“Package on Package”).
It’s a communication issue. All of the ICs in a phone don’t communicate over a single bus – almost every one ties directly into specific processor pins. This would restrict block size and placement.