I very recently bought someones stash of fabrics which date back to the 1990's through at least 2003 (based on dates on selvages. Some of the fabrics are thinner than others, some have a kind of brittle feel to them (even after washing).
This is exactly what I experience when shopping for fabrics now, and it doesn't matter where I shop. I buy what I like and what feels good to me. Of course, as with all of us, my purchase choices are influenced by price. However, if I don't like a fabric for any reason I won't buy it at any price.
Having said that, it's possible that kit makers use 'lesser' fabrics in the kits simply because you can't touch or feel the fabrics beforehand, especially if you buy online.