Seems like a cool website, added to my list thanks!
CSS
Why not use SASS?
Let me reverse that question and ask: Why use Sass?
Sass is lovely. I love being able to split my CSS into multiple files in the same way I would split the rest of my code. It's really nice for making more maintainable CSS on larger projects.
My portfolio would've been hell to make without SASS helping with the organization.
CSS can already be split into multiple files though.
So why not just use CSS?
Sass is to CSS what jQuery is to Javascript.
It has variables as well as modules, easily nestable definitions, and inheritance so that you can have base classes
CSS has variables, modules and nesting.
By inheritance, do you mean the extends
keyword? Because if so, it just seems like going further down the misguided BEM path instead of picking better selectors.
The same reason you don't start new projects using jQuery.