I'm with you on this one.
"Back when" we had a cog board w/ all of the individual cogs and spacers to build a custom FW or simply replace a worn single cog for Sun Tour and Shimano.
Small cogs wear faster than large one making an entire FW/cassette replacement necessary when one 15T would keep it in service for another season or two easily and inexpensively.

Those days are gone.
It's part of the disposable parts-changing mentality where sets of cassette cogs are pre-pinned to keep parts from being installed upside down or spaced wrong in the tight tolerance of indexed systems. Worn 15T? New cassette time.
Lawyers & inventory control experts at work.

-Bandera