I have found massive differences in fabric across various reputable brands over the 12 or so years I have been quilting. Some are thinner/silkier, others are thicker and not very smooth. I assume the various manufacturers, maybe even designers, have their own specifications. There has to be a reason for the differences, and the decisions have to be made somewhere.
I agree with Lena1952, I have some of what I identify as older fabric (based on the style of the selvage), and that often seems extremely thin, so I don't necessarily think standards are dropping. An ice cream manufacturer will make the carton smaller so they don't have to raise the price, but I think they will be less likely to lower the quality of the product, because then people will buy a different brand.