Someone shopping for an M3 screw would not be interested it trying out a smaller one, but research has shown that people are interested in trying on a smaller size with clothes ;)

However, clothes are nearly impossible to simultaneously standardize and commoditize: people (and their feet) come in all shapes and sizes, so different brands in particular, but also different lines of those brands, will have different proportions.

As a tall guy, I hate that clothes keep growing in width with larger sizes, but rarely grow in length (pants/trousers included), but only because you can't get things without trying them on in multiple sizes.

If you do not understand why standardized sizes are not mandated or something similar, just look at any bespoke garment creation and all the variable measures being taken for a shirt, pants or jacket — just make realistic combination of all of them in different sizes, and you are looking at ~1000 "standard sizes" or more.