Microprocessor

Only two?

Some of my current devices have almost 100 ground pins.

The usual reason is current density - a pin (and particularly the bonding wire to the chip) can only handle a certain amount of current, and apart from that the inductive effects of large current pulses play havoc with noise immunity. (If you are interested in this issue, google 'ground bounce'). More ground pins-> less current per pin, less bounce per pin, nicely distributed ground on the actual die.

Cheers

PeteS