If they did that, they'd have too many sizes. Just look at jeans: most brands carry 100 or so "sizes", not including variations (tight, slim, regular, loose, baggy, tapered, straight, etc). With women, it's even worse because their body types are even more varied. It's easy to end up with 1000+ variations if a store tries to cater to everyone.

Instead brands cater to certain demographics and their sizes are relative to the body types they try to attract. It's unlikely that a woman who regularly buys from NY&Company would expect something at Lane Bryant to fit them.

In Europe they mostly have, but it's still so variable and subjective. For one thing, people are very different sizes, based on your market. I mean, I'm a "large" here (in Europe) but a "medium" in America, presumably because the average American is considerably fatter than a European of similar height.

And tailoring/fashion add another layer of subjectivity. A pair of size 34 jeans are much tighter in Europe than the US, because that's de moda. And there's considerable variation between brands, depending on how they want you to look when you wear their stuff.

In the end, there's just no getting away from the fact that you'll need to try clothes on if you want a good fit.

In what way would they benefit from it? In general, they'd rather encourage ignorance-based brand loyalty. In addition, this way brands can compete on vanity sizing.

"I'm a 12 in Brand X, but an 8 in Brand Y! I'll take Y!"