Every manufacturer has slightly different sizing, so a 56cm Specialized is not quite a 56cm Giant. Given that there are literally dozens of companies making excellent frames, there isn't a huge need for that many frames. Colnago has many more sizes I believe. Colnago can afford to do more frames because the lugged construction is much easier to vary than a monocoque frame. Ultimately, since you can swap stems, it just doesn't really matter. I ride a 57cm frame, with the correct stem, I can find the same position on a 54cm frame.

If you're having that much trouble finding a bike that fits, I suspect something else is off.