If dress sizes really were uniform across all manufacturers, it would be even harder to find a good fit. It might even be impossible.
As it is, shops profile their customers, so if you're the target market for Shop X, there's a good chance they can fit you. If you have some difficult sizing feature like a large bust, small waist, long legs or whatever, you've got a better chance of finding your fit if you know which shops cater for you. If everywhere were completely uniform, nowhere would cater for your specifics.
I share everyone's frustration as I have a hard shape to dress as well, but it just isn't realistic to expect to be able to walk into any old high street shop, pluck any mass produced item off the peg and expect it to fit you specifically. You may complain that, say, not everyone with wide hips has a large waist and you'd be right, but overall, if one body part is larger, the rest are likely to be larger too. It's just statistics.
My advice is to learn which shops suit you, prioritise quality over quantity and befriend a local dressmaker for alterations and commissions. Even made to measure stuff doesn't have to be madly expensive and if it fits you perfectly and will last ten years, it's worth it.