It is indeed true that we have a lot of supermarkets in Austria, which might make logistics more expensive. But nobody ever explained why we need so many supermarkets from just 3 chains (REWE, Spar, Hofer - everything else is neglectedable). My only explanation is that the big 3 want to squat every possible spot to keep competition out.
The other argument from the chains is that they are forced to buy brand products from Austrian distributors that are more expensive than German ones due to lack of volume (think of making a wholesale contract for M&Ms with Mars Germany, not Mars Austria). Again shady argument, because they could have sued against these practices years ago.
Germany has the luck of being a big country with scale effects and healthy grocery store competition.
Do you suspect there is some other reason? Like, they can get away with it in Austria but not Germany? What’s a “non-shady” explanation in your eyes?