Why are GND pads often only connected by four traces?
This is to make soldering easier. Those 4 traces make it easier for the pad to heat up as the heat can only escape through those 4 traces. Soldering to a GND pad is difficult enough as it is, but if you had none of the spokes, you would essentially be trying to heat up the whole ground plane. It would be exceptionally difficult to heat it sufficiently to solder.

There may be some other reasons too, but this is the one that I know of!

As pointed out by @John Go-Soco in the comments, these are also reffered to as thermal reliefs