Men's shirts button with the right side over the left, and women's do the opposite. The convention is universal, arbitrary and has survived every other change in how clothes are made.
The explanations point to different origins
The most commonly repeated account holds that women of means were dressed by servants, and a mirrored arrangement suits someone facing the wearer.
A second account concerns weaponry: a man drawing across his body with his right hand benefits from an overlap that does not catch.
Both explanations describe a small, wealthy population, yet the convention became universal, which suggests imitation of high status clothing rather than practicality.
Conventions in clothing outlive their reasons
Once a convention is established, changing it produces garments that look subtly wrong to everyone, without offering any visible benefit.
Clothing is judged largely by whether it matches expectation, so a detail with no remaining function is still costly to abandon.
Vestigial details persist across menswear and womenswear for the same reason, from working cuff buttons to lapel buttonholes that no longer fasten anything.
The pattern and the factory reinforce it
A garment's handedness is built into the pattern, the cutting layout, the interfacing and the placement of buttonholes.
Machinery for sewing buttons and cutting buttonholes is set up for a given orientation, and a production line running one way is not casually reversed.
Reversing a garment therefore means new patterns and reconfigured operations, so the default stays the default indefinitely.
Practical consequences are minor but real
Right-handed wearers generally find the men's arrangement marginally easier, since the working hand manipulates the button rather than the buttonhole.
The difference is small enough that most people never notice, which is precisely why there has been little pressure for change.
Zips and other fastenings sidestep the question entirely, which is one reason the convention now applies mainly to shirts, blouses and tailoring.
The convention is eroding at the edges
Unisex and genderless ranges have to choose one orientation, and many simply adopt the men's arrangement for the whole line.
Some brands treat the detail as a design decision rather than an inherited rule, particularly where a range is not split by gender at all.
Where categories remain separate, the convention holds, which suggests it is maintained by the structure of the industry rather than by any preference of the wearer.