As noted near the end of Brisingr, Eragon learns that Galbatorix gains his power from
the Eldunari. For every Rider he killed during the Fall, he gained more power.
So he must be incredibly powerful. Why didn't he just confront the Varden, elves, and dwarves directly and butcher them, instead of waiting for them to combine armies and march on Urû'baen?
In Lord of the Rings, when he could, Sauron attacked Isildur's army before he got his ring got chopped off. In Star Wars, Vader did go into combat every once in a while. Why wouldn't Galbatorix do the same?