Its a big field bog, and the people in charge of harvest have to move around in it. Also they drag the boom across the bog surface to harvest, so poles in the ground aren't really an option.

The better question is... why not float rafts on the bog with the berries to let the spiders flee into those first then let the people wade in. Arachnid employees deserve safe evacuation! Probably wouldn't fully solve the problem, but might make it a bit better.