J.W.
Actually it isn't the dress makers, you are just not average. I have the same problem on top. My exes sister has a large butt, she used to give me all her dresses and I would alter them to be even. Basically taking them up 2 inches in the middle of the front and tapering it around.
It is not about being model thin and having no butt, you will find most people don't have this issue. It is frustrating, I know, my daughter got her dads butt. :p