Starfleet totally cheaped out on the fabric quality, jeez
The TOS Starfleet uniforms are actually a good example of the Sam Vimes ‘Boots’ Theory of Economic Unfairness. (Namely, a poor man can only afford to buy cheap boots that wear out quickly. A rich man can buy good boots that last ages. Over their lifetimes, the poor man will actually spend more on boots than the rich one.) Their budget only allowed for the purchase of cheap velour that began to look like crap after a couple of washes and had to be replaced. They never had enough money at one time to buy better quality fabric.
To have Sam Vimes used to explain Star Trek costuming has made my night.