I would presume the point is that the box shifters have a model more like tesco. Stack em high and make up on volume.
The local shops are more like traditional shops - experts in their fields, providing a lot of value added services and participating in events and supporting the community. The smaller scale model means you need to make margin on everything you sell and can't afford to simply sell at loss or cost + a point to clear shelves.
This argument has appeared a few times and while not seeking to defend the local shops, it's enough to say I highly value the services and expertise they provide and have never bought anything from a box shifter for that reason.