Well, depending on who you ask, vanity or capitalism.

Women’s sizes were chosen.

Men’s sizes are typically measurements, usually waist × inseam and neck × sleeve × chest

Women’s sizes are harder because they have a wider range of measurements needed to get a good fit. Bust size, waist size, and hip size can have different ratios to each other, even when the waist size is the same. So, Wanda might have a 20 inch waist, a 58 bust and a 36 hip measurement. But her sister June might be 20 waist, 36 bust and 58 hips. Obviously, clothing measured the way men’s is wouldn’t give as reliable a guide.

So, way back when mass produced clothing came onto the scene with standardized sizes, something needed to be picked.

Turns out that a size 6 sells better than a size 20, even if the actual measurements are exactly the same. Not that men’s clothing is immune from vanity capitalism, you should see the clusterduck that is XL sizing.

But, with “dressy” clothing, mens shirts are usually going to be measured. Women’s sizing, particularly dresses and pants, they go by the fairly arbitrary numeric system based on ratios. Just don’t ask me how it was calculated originally, I never cared enough to find out.

Thing is, while those sizes were originally meant to standardize things, that no longer works. You go get a size 6 in one store, hold it up against a size 8 from another, and they’ll be the same measurements. Why? Because they’re playing a numbers game based on vanity. Some places, a size 6 is unrecognizable as an actual size, it’s just so far off from the median.

Also, I use size 6 a lot because it was, at one point, the “default” size for models and mannequins. I think that’s changed, but it stuck in my head, so I tend to pull it up as a baseline example. I know it’s usually what’s used for fitting models, which is a whole thing of its own. It varies a lot more nowadays for runway and catalog work though. Height is more important in catalog work afaik.

Anyway, tangent aside, shoes are a bit more practical. Women’s feet have a slightly different set of angles, so just a toe-to-heel measure wouldn’t work exactly the same between men’s and women’s feet. I can’t recall the exact points where measurements occur to get the different sizes, but that’s what it comes down to. You have to measure the feet differently to get a good fit.

Which is the overall why none of the clothing sizes will cross over well.

Yeah, a men’s XL is going to fit a woman with a given bust measurement about the same as a women’s xxl (iirc, don’t hold the exact conversion as fact, I’m just pulling from memory here), but they may not fit the same.

A men’s dress shirt is going to fit a woman horribly, even if it’s the right chest or neck diameter. It’ll be cut for a bigger waist, with longer arms. But a woman’s dress shirt will fit z better*, because that’s taken into account.

Funnily enough, men that lift a lot of weights end up having trouble fitting men’s clothing sizes as well. You get something that fits your chest, it won’t fit your waist (unless you’re a power lifter, where you tend to see less difference between chest and waist than in bodybuilder circles), and it may not fit your neck worth a damn. Buy for the neck size, your sleeves can be baggy.

The patterns used don’t scale up the same as the human body does as it puts on muscle. It’s still not as big of a pain in the ass as it is for women with significant differences, but it is a pain in the ass lol. I’ve never been able to buy a suit off the rack. I’ve only had a few, but they all had to be tailored.