And it has to do with having no inventory or stockpiles on the shelf, but items arrive as you need to build your product. What that means is that it's much more difficult to actually find stockpiles of already built weapons. ↗
CD stores have the disadvantage of an expensive inventory, but digital bookshops would need no such thing: they could write copies at the time of sale on to memory sticks, and sell you one if you forgot your own. ↗