Men's clothing is still sold in inches while most other clothing is sold in abstract sizes. The difference is not tradition for its own sake, it follows from how each system was originally built.
Tailoring measured the body, not the garment
Before ready-to-wear, a suit began with a tape measure and a customer standing still. The tailor recorded chest, waist, sleeve, inseam and a dozen smaller figures.
Those numbers described a person rather than a product, so they carried meaning outside the shop. Any competent tailor could work from another's measurements.
When manufacturers began cutting in volume, the fastest route was to keep the vocabulary and simply produce garments at fixed intervals along the same scales.
The chest measurement anchors a jacket
A jacket size refers to the wearer's chest circumference, not the garment's, and the maker adds ease on top of it so the coat closes and moves.
How much ease is added is where houses differ. A classic cut may add several inches through the chest while a slim cut adds barely any, from the same nominal size.
The label therefore tells you which body it was drafted for rather than what the finished garment measures, which is why two jackets marked identically can fit very differently.
Shirts kept two numbers because collars are unforgiving
A shirt is sold by collar circumference and sleeve length because those are the two dimensions where an error is immediately visible and cannot be adjusted by posture.
Body width is left comparatively loose and handled by fit names such as classic or slim, since a shirt is worn tucked and gathered by a waistband.
Collar sizing survived even as shirts became more casual, largely because the alternative would have thrown away the one measurement most men actually know.
Trousers kept the waist figure and then drifted from it
Trousers are sold by waist and inseam, both nominally in inches, which should make them the most literal garment in the wardrobe.
In practice the stated waist has drifted upward in generosity over decades, so a labelled figure often measures more than it claims when a tape is put across the waistband.
The drift happened gradually and unevenly across brands, which is why the number remains a useful starting point and a poor guarantee.
The system survives because it degrades gracefully
An abstract size conveys nothing when a brand's scale is unfamiliar. An inch measurement is at least anchored to something physical, even where it has been flattered.
It also supports alteration. A tailor working from a garment marked in inches has a reference point, whereas an arbitrary size number describes only itself.
That practical advantage is why menswear has resisted converting to a purely abstract scale, even as the measurements it prints have become approximate.