I mean, I get that it sucks online, but in person? Who cares what the label says? I'm an adult. I can easily tell by looking at a garment how it's going to fit me.

That said, if we could just get the critical measure online that'd be fantastic. No need for sizes, I know how big inches and centimeters are.

And, as it turns out, my favorite retailers do in fact include measurements, but I'd rather have a few quality items than lots of garbage, which is also why I own a sewing machine because sometimes I really love a dress but the manufacturer doesn't accommodate my specific frame. I developed this practice when I was broke and shopping out of thrift stores. It allowed me to buy almost anything and tailor it to make it fit. Really broadens your fashion horizons.

With regard to why sizing is difficult, I'd guess it's just consumer laziness or cognitive dissonance. Although it's maybe a little bit of efficiency too. How many models should I produce (and how many lines do I have to run) to fit every woman just right instead of lying to all of them? For pants alone, if you really want it to actually fit, you're going to need ankle, calf, knee, thigh, inseam/outseam, glute, hip, and waist (and crotch to waistband if you're offering different rises). So if you've got even just 5 measurements (probably not enough as no way do all women fit tailored within 5 different calf sizes), you've got 5^9 different products (and therefore machine configurations) to cover just that space, because yes there are women with massive calfs and small thighs or same waist/hip or whatever combination you can imagine) and that's all just for literally one style. If you've got five different pants that's immediately 5^9.

Lots of (american) women are perfectly fine with their 36in underbust but would be shamed to admit they need a 46in hip with their 32in waist for all that ass. Much better to just lie and say I need an 8 which will not in any world ever make it over my butt.

Maybe we can compromise on a 'call it' measurement which is on average 2 less than the prevailing standard would suggest. If your countries' system would have you in a 8, you can 'call it a 6', and then we're all happy.