Sorry, but I like the simplistic power of CSS. I don't know why everyone keeps trying to extend and build on it. Guess I'm in the minority.
See the answers given below.
These types of tools are for large scale projects. No developer would use them for a 5-page project unless he's really insane.
Can somebody explain why this amount of work and tooling is necessary to style a website?
Because as the requirements of a site increases, the ability to safely and confidently add new features becomes more and more difficult, meaning tools and techniques like this quickly become needed. That being said the work OP is talking about is very much at the fringe, where a lot of front end people are rethinking how things have historically been done. The rule book (if there is one) is being ripped up continuously.
One of the most difficult tasks in creating CSS for -- large -- websites is naming elements and structuring files and folders. In a way everybody understands and is able to extend it.
After solving the cross browser compatibility and performance issues with PureCSS / Autoprefixer / Gulp the next frontier is to find a way for a scalable CSS architecture.
Today we have BEM / SASS as a best practice with contenders like web components and the approach presented in this article.