Am I missing something here?
Usual disclaimer here, Im not a pro sprinkler guy, but Im a pro at some other stuff.
The flow is whats important... if you add up all the area inside the pipes that are full of water, that is going to add up to a lot more area than you might think.
A comparable task to blowing out the irrigation system, would be maybe to blow out a couple of huge tractor tires full of water, with only a few small holes for the water to escape from..... It will take a while with the home type compressor.
Pro's dont have that kind of time to spend on a blowout. They need the big one in order to offer a competitive price, and Time (labor) = Money. Lots of it.