Whether the tool should or should not be necessary is an entirely different question. The reason it is necessary is because it enabled writing "CSS" in a way that it was never meant to be written. And as evidenced by the blog post and all of the discussion here, adding the functionality to browsers isn't at all straightforward.

So a better question is why shouldn't the tool be necessary? It's all about writing the same CSS in a more developer-friendly way, not enabling CSS to do something it never could before (like variables and flex did). Why should browsers have to worry about this added complexity? Just write your stylesheets however you'd like, and then run a tool to compile them down to standard CSS.

When I look at these proposals, I would still prefer using Sass over regular CSS. It's syntax is just cleaner.