How are BGA dies constructed?
Yes, BGA packages are like little circuit boards. On high pin count devices, in nearly all cases, the balls that lie directly under the die are mostly ground (and sometimes power) connections. The ground balls are fed directly through to the substrate of the die, while the power balls connect to internal power planes.
Also, because of their direct metallic connection to the die, the ground balls help remove heat from the package.

All of the I/O is connected to balls near the periphery of the package, keeping their internal traces &mdash; as well as the PCB traces they connect to &mdash; shorter and inductances lower.