Modern CSS has _some_ of the features of SCSS/SASS. It does not have all of them. But most importantly, many of dependencies one might want to use also make use of SCSS/SASS downstream. If you're happy to build everything from scratch and eschew any dependencies that require a build system, then have fun explaining to your product person why it took so much time to build a thing that they know very well is a pre-built component in some frontend library somewhere.