In the "THe VILLaIN aRC oF VANiTY SiZINg" section, vanity sizing is framed as marketing strategy which is successful because of the psychology around that - linking out to https://www.sciencedirect.com/science/article/abs/pii/S10577... for more detail.
It certainly wouldn't be the first time the most profitable marketing strategy is unrelated to aligning with what's optimal for the consumer.
Of course education could help about this and other psychologically manipulative tactics by corps but such kind of education is heavily frowned upon for being seeing as anti-capitalist and (more propagandistic) as un-american, so there is zero of such kind of education.
Translating the confusing science speak, basically:
Appearance self-esteem takes a hit when they don't fit in a size. They take it out on the clothes: "I hate their stuff, they suck." They buy more of other stuff to compensate for the hit, whether non-sized accessories (I am pretty) or book/tech (I am smart even if I don't fit).
People confident in their appearance are immune to the effect, and simply think it's sized wrong or runs small.