You can be in perfect shape and still not find clothes that fit. The issue IS the sizes.
Normalize going to a tailor, instead of grumbling about how you aren't benefiting enough from the sweatshops mass retailers are running.
They're doing it because people are buying clothes based on superficial appearance, and most people prefer the aesthetics of the hourglass shape.
Rectangular clothing doesn't sell as well because it doesn't look as good on a mannequin even if it fits better.