In rigorous academic contexts things that are generally accepted or common knowledge do not need to be cited. Nobody is disputing the existence of frozen milk or sliced bread for sale. They are commonly known and accepted facts. We do not need a peer reviewed citation for those claims to be believable.
The claim is not that sliced bread is always available in every place on earth. Nor is it that bread is usually sliced. That is a different claim that you have added. The claim is that sliced bread is available for sale, nonspecifically. It is a commonly known fact that bread can be purchased sliced in some places.
If you make a specific claim, like: sliced bread is not usually available in Nordic countries, I could see that needing a citation. You have made specific claims about sliced bread availability and market preferences. I am making the general case that it exists.
The point is that there is some basic level of knowledge that we don’t need to validate with a third party source. People drink water. I’ve never seen a scientific study or source for this, but it is universally accepted as truth. Bread is sold sliced sometimes is a similar fact.