Why are there so many grounds?
Because wires and traces are not perfect. They all contain some inductance which impede high frequency currents trying to flow through them.

Needless to say: Radio frequency = really high frequency.

The bad effects are increased noise, voltage spikes when the chip's current demand decreases and voltage dips when the chip's current demand increases.

So what do you do if are trying to move lots of water and all you have are tiny pipes? You use a bunch of tiny pipes in parallel. Parallel inductances results in an overall lower inductance.

Also, because IC packaging is standardized and if you don't need all those pins, you might as well connect them to ground because it reduces noise due to the aformentioned high frequency currents trying to flow through trace inductance.