Can you give any more insight as to why? Designers always seem to want to build design libraries, I don't understand why they wouldn't want to use a common API as the basis for that so that there's a contract of sorts with developers.
I would assume their thought is, that it limits their creative spirit and in general is too technical. But they start (as 2 people teams) on figma a complete own design lib with a lot of variables and custom components which look the same like any other lib.
Things like Tailwind take the core concepts of bootstrap and build and improve on it on a grand scale. It is like comparing jQuery (bootstrap), to React (tailwind).
People really do sell Tailwind as if it’s a Bootstrap-like framework. I think that’s why so many sites (see: most startups) have a very similar “Tailwind look” now - everybody wants to use Tailwind because it’s hip, but nobody actually wants to write styles if they can help it, so they just copy-paste a very basic rounded-border-plus-shadow style onto all their components and call it a day. It’s a little better than the “Bootstrap look” because at least it doesn’t come with a default accent color that nobody will ever change, but man, it’s not that much better.