I'm not sure why utility frameworks are so popular. It seems like Sass solves most of the problems of vanilla CSS. Writing Sass is not only quicker and more powerful than regular CSS, but you can also create your own utility classes very easily with loops.
Moving CSS into HTML does not makes much sense either, as it's far stylistically cleaner to separate CSS and HTML into their respective files.
Plus, if you have two elements with the same or similar styling, re-writing all those utility classes over and over seems extremely cumbersome. Why not just write the class name and use it everywhere it needs to be used? .button or .button-primary is far simpler than writing out inline CSS over and over again.
Moving CSS into HTML does not makes much sense either, as it's far stylistically cleaner to separate CSS and HTML into their respective files.
Plus, if you have two elements with the same or similar styling, re-writing all those utility classes over and over seems extremely cumbersome. Why not just write the class name and use it everywhere it needs to be used? .button or .button-primary is far simpler than writing out inline CSS over and over again.