It's a good marketing ploy. People buy clothes that make that feel good, and 32" feels better than 36".
Also, designer cloths deliberately limited their clothing to a target market, which didn't include the fuller figure. So the mass market copied them, so the customer could say, 'I can't fit in xxxxx's jeans, they don't make them in my size. But I fit the M&S copy in the same size I can't fit into there'
But even that is going slightly by the wayside, as the designer's target market in starting to expand around the waist, and their sizes are becoming slightly more generous.