Breast is best is actually just a really clever marketing campaign, but not originally from the people you might expect. In fact, it's a brilliant example of a very common marketing technique - and once you are aware of it, you can spot it around you on a daily basis.
Let's take chickens (a leap I know but bear with me) a normal life for a kept chicken is really quite simple. A hen house to keep them safe at night, an area for them to move about, scrat and eat grain, water to drink and you're pretty much done.
Veer away from these basic requirements and as you might expect you have problems. As Hugh Fearnley-Whittingstall has highlighted with his "chicken out" campaign, remove their outdoor wandering/scratting area, coup excessive numbers together - and you get unhealthy, underweight chickens that need large amounts of antibiotics and other drugs to keep them alive (with some companies injecting eggs with antibiotics before the chickens have even hatched!).
So in short, give a chicken a standard chicken life and you end up with a normal chicken. Treat it in an abnormal way and you end up with a chicken packed with antibiotics, excessive salt levels, flavourings and water.
Let's picture accurate labelling of these products. The first - standard chicken, easy. The second hmmm "substandard battery chicken; includes antibiotics, salt, flavours, reduced protein and increased fat levels".
I'm thinking the second isn't going to be a big hit in store.
Retailing is about pounds kerchinging into the till, not accuracy - and so the marketing bods step in. The normal chicken is elevated to "best" and they give it a cool name like "free range", "organic", "finest" or "taste the difference". Lots of different ways to describe a chicken that has in reality, only had an existence which at least met the basic requirements.
Supermarkets sell more "standard chickens" than any other type - and not just because of the price tag. In fact, even if the price difference was nominal, sales would still be higher. This is because the vast majority of people believe standard is "OK". Fair enough it might not have quite as much flavour as the "fancy one", but then they didn't home grow the carrots or make the gravy from scratch either. OK does most of us pretty well and this is what the marketing people understand! But let's not forget, in reality it's not standard, it's substandard that is cleverly marketed.
This happens in all areas of retail - tomatoes ripened on the vine, where they are supposed to be and thus actually taste as they should, are "finest". Those picked early and ripened in boxes during transportation leaving them more like tasteless balls of water are "standard". Sausages that have more meat than fillers are "finest" and so on and so forth.
So back to breastfeeding.
Breastmilk is the bog standard, normal substance a human infant is built to consume - there's nothing best or superior about it. A survey a few years ago asked mothers whether they would buy a "value" formula, unbranded and in plain packaging - rather like the blue and white stripey budget tins at the supermarket. Absolutely not was the overwhelming response - the perception was that value branding compromised on quality, whereas standard ranges are "OK". It might not be "best" but it was fine.