My local Aldi normally has a box of boxes in most aisles. My local Tesco has it (unhelpfully, considering how many tills there are) near the exit.

I suspect that part of it is that many of the boxes are now trays designed for shelf display. The shelf-stacker just unwraps it and puts it on the shelf. This also makes the boxes slightly less useful for putting your shopping in, as the original contents were held in place by plastic wrap.