Markets did spring up around ancient armies. See Kautilya’s Arthasastra, Sassanian “circle of sovereignty,” Chinese “Discourses on Salt and Iron”, they show “that most ancient rulers spent a great deal of their time thinking about the relation between mines, soldiers, taxes, and food. Generally they realize creating markets help not just feed soldiers but also helped them get a lot of other stuff out of their people too.
They used to need royal estates or workshops to produce things, or to “requisition” it directly from workers. But now they can just use markets to incentivize production of what they needed.