((shrug)) Tailoring or bespoken. That's pretty much it.
They choose models based on how they fill out the clothes. You are choosing clothes based on how you fill them out. The only way to change that to make things always perfect on you is to have the clothes made for or tailored to you.
You are clothes savvy. I know you'd never think honestly that you should be able to walk into any store and pick up any size (fillinblank) item and put it on and it would be perfect. That doesn't make sense on a macro scale because it wouldn't accommodate all the women out there who are the same size but aren't built like you. So why think that it should work honed down to the super micro version, which is ultra perfect fit and styling?
For every time that you're frustrated about this, think about all the women who dream of looking like you do in your clothes. Or of having your style. Besides, if it was easy? You'd be bored out of your skull and you'd be looking for something else to learn and perfect. You aren't a girl to rest on your laurels. You like a challenge.
ETA: Just read Janet's link. Awesome. Exactly right. Blunter and more straight-forward than what I was saying, but the same thing. No one really looks like that. Everyone works at it. For whatever reason, I've always thought of tailoring automatically. Maybe because I always played around making things with a sewing machine but no lessons?
FWIW, a friend was almost dying laughing when she tried on a bikini and I told her to have the top (too large for her) altered. Then I showed her where it would be taken in, creating a smaller size, but also a more pronounced curve to the top, and told her to get it lined. It was a huge moment for her. Suddenly she realized she could feel good in the bikini. "Like a real girl!" The irony? She's 5'11 and a perfect model body type.