As a 5'10" woman, I do find it oddly convenient a lot of the time as that's seemingly the height of 99% of models, however everyone has different body proportions so it doesn't end up being a super useful metric.
Leg to body ratio can differ greatly, and when buying tops you can really only compare the fit with people who have a similar torso length than you. No clothing site is going to start listing individual measurements of their models like that, so it'll always be a vague guide.
I actually spoke to someone a few years ago who worked in marketing at a US clothing manufacturer (I'm Australian so not sure how well it applies to my own experiences), but apparently many stores are trying to encourage a small portion of returns of online orders because many people end up buying more items in the process.
My personal theory here, is that sites are just trying to let people know that a model is wearing their clothes. It's not to help gauge sizing, it's to let you know that person is _really_ a model. Models are often tall and slender, so they want you to know that tall and slender people look good in their clothes. The effect of "If an attractive model would wear this clothing, you can look like a model by doing it too!"
Companies which use ordinary people in ordinary size ranges are much easier to trust and buy from. Companies who set up unrealistic body proportion goals make it hard to know what "fit" is available.
A niche class I've found which seems to be appropriately dimensionally accurate is mail order leather jackets. They give you as much help as possible to understand fit across the body not just S/M/L. Maybe it's motorbikes or maybe it's the return business from happy shoppers, or maybe it's the high unit cost.
A now dead regional Australian clothes and footwear company (rivers) used to use staff as models. All shapes and sizes, all ages. I felt I could relate to the fat guy from marketing in that shirt, the same with an outfitter called Lowes who used ageing rugby league stars, who generally grow a beer gut and carry a lot of ex-muscle. I knew a shirt from there was going to go with my bald spot just fine because Wally Lewis modelled it and he had the bald spot from hell. I didn't have the muscle mind you but that's on me, not them.
I have the same problem and I'm only 5'11" (177 cm). Size "Small" might fit at the neck/shoulders but with lots of extra fabric around my midsection, but most retailers don't carry much inventory for Small, and almost never have Extra Small. In the SF Bay Area, the small items are always out of stock.
At one point I found that clothing boutiques in the Castro tended to stock clothes for skinny guys, but those stores disappeared during the pandemic.
Now I have to go shopping in Seoul or Tokyo or NYC, but if the pants fit in the waist, they're usually too short.
