Because that would be a very short term solution for the armies, merchants would very quickly stop following armies which robbed them of their livelihood. As pre-modern armies didn't have sophisticated supply trains to provision their soldiers with anything more than the bare essentials, they relied on these merchants for anything from better food and alcohol to sexual services. The short term gain of robbing the merchants once doesn't outweigh the long term detriment to quality of life and morale from soldiers losing their expected supplies and distractions for the remainder of the campaign.