The sizes vary within brands themselves.
Traditionally the solution was to get a tailor take your measurements and make a pattern for you and cut from cloth to make you a garment. Obviously, in today's day and age, that gets a bit expensive for most people.
https://www.indochino.com/suits/fit
That one is a "measure at home" affair, but I've also heard tell of ones where there's a pop up at a co-working space or whatever, and you go get measured, pick your fabrics, and get the shirts in the mail a few weeks later.
Cost-wise, it's definitely more than getting them at a department store, but it's not ridiculously more. Think like 2-3x rather than 10x.
That said, a tailor will make you pose properly and not slouch or droop your shoulder, so they would take better measurements. Presumably, SW could correct for bad posture as well.
Manufacturing at scale is currently impossible.
Tailoring is basically robotics' worst nightmare: input material with varying characteristics from batch to batch, delicate materials, multiple different motions and tasks, a non-reducible movement and orientation space (e.g. sew a hem along a curve), and a complex mapping between physical tasks and end result (different stitch on seam = different motion).
We'll get there (and if it weren't for low cost labor, would already be there for t-shirts). But it's really hard.
I also think what you’re describing could be easily solved with more measurements of clothing you’re considering.
The issue is it just doesn’t scale well abd is limited by types of garments on offer, no standardized way to share measurements with other manufacturers, and requires we redo the process or for me to ‘guess’ at adjustments if my body changes in any material ways - they only come to major cities once a quarter or so.
The GP is more referring to companies who deliberately don't conform to standards. Best example of this is women's clothing that alter sizes so they are more appealing to women who don't traditionally fit that size number.
TL;DR - "Omg I finally fit in a size 2 dress" is a huge selling point, even if its not categorically true.
But as someone who is on the shorter side with an athletic body type, almost no clothing produced today (aside from high end made in US brands) fits me at all. Vintage stuff does. No amount of standardization will get around that. I have to pay more for tailoring, getting custom made, or spending tons of time searching out boutique brands that work for my body type.
What I’m suggesting fixes it all.
Some companies do try to address this to a degree with slim or athletic cuts, and yes a tailor could take you the rest of the way ‘mostly’ there, but the future of clothing is 3D scanning and printing.