The full problem faced by Giddianhi is not apparent until we read verse 6. Of course the arable land was available. The problem was not in the availability of land to be farmed, but in the farming of it. To farm, the Gadiantons would have to spread their forces thin, and they would be susceptible to raids from the Nephite stronghold. Thus the one viable method of obtaining food was not available to them. Of course even should they succeed in raising any food, they were an entire growing season away from eating it.
The solution preferred by Giddianhi was to attack. If he were successful, his problem would be solved for he would have access to the stored food in the Nephite stronghold. Since it was his intent to conquer in any case, it was the preferred solution.