It has to do with the magic involved in the harvest… They have to put their symbols out and in the open for the magic to work properly. If you don't know what your looking for you walk right into their trap. Now that we have had so many "harvest parties" we can accurately see what it takes to make the magic work and be successful. They have access to ancient black magic. They don't understand/ can't successfully use rune magic. IMO the darkness has for the first time, in a long time, lost control and I think this is why. We are awaking ancient magic on our side.