A men's dress shirt is sold by neck measurement and sleeve length rather than by a single number. That system survives because the collar has almost no tolerance while the body has a great deal.
The collar is the tight constraint
A collar buttoned with a tie has to sit against the neck with a small, consistent gap. A little too tight is uncomfortable and a little too loose looks sloppy.
The usable range is narrow, which is why neck sizes are offered in half-inch or single-centimetre steps rather than in the broad jumps used elsewhere.
No other dimension in a shirt is judged that finely, so the collar became the dimension the size label reports.
The body has generous tolerance
A shirt worn under a jacket is judged mainly at the collar and cuff. The chest and waist can vary considerably before anyone notices.
Traditional shirt cuts allow substantial ease through the body precisely so one pattern covers a range of builds at a given neck size.
That tolerance is what makes a two-number system workable, since the neck and sleeve carry the precision and the body absorbs the variation.
Sleeve length is the second fixed point
Cuff position is visible against a jacket sleeve, and the accepted band is narrow. A cuff that disappears or overruns is immediately obvious.
Sleeve length also varies far less predictably with chest size than height does, so it cannot be inferred from any other measurement.
Reporting it separately lets one neck size serve people of very different heights, which a single-number system cannot do.
Casual shirts abandoned the system
A shirt worn open at the neck has no collar constraint at all, and its sleeve is judged loosely against the wrist rather than against a jacket cuff.
Once both precision points disappear, the shirt can be sized like a t-shirt, and small through extra-large is entirely adequate.
This is why the two systems coexist on the same rail, and why moving between them produces confusion about what size someone takes.
Slim fits reintroduced the problem
As body-skimming cuts became standard, the ease that made neck sizing sufficient was removed, and chest and waist became constraints in their own right.
Brands responded with fit names layered on top of the numbers, so a shirt is now described by a neck size and a fit category that has no defined measurement.
The result is a labelling system carrying one precise dimension and one vague one, which explains why the same stated size fits so differently between brands.