The number of tailwind classes is so large now that copy/pasting the whole set into your project would mean a huge bundle size.

Part of that the tailwind package is doing is making sure that only the relevant tailwind classes are included.

So Vite builds a custom style sheet based upon the Tailwind classes that are actually used in the project?

yes, exactly

x86 ASM is text but we don't build software by copying and pasting it. While the underlying thing we ship is CSS, that doesn't mean we can't add tooling layers to make it easier or more efficient. While it's totally allowed to write by hand, most frontend web developers I know are very comfortable with NPM, and indeed having dependencies consumed from a package manager is often preferred.