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Some tooling is for managing the complexity caused by complexity of the site/app.

Some tooling is managing the complexity of managing component-centered design (particularly scoping); it's a very positive approach to design but it's not without its problems.

Some tooling is to help developers avoid having to actually learn the CSS language well and/or to force it into JavaScript.

> CSS is essentially platform-specific due to differences between chrome, safari and firefox.

That's an exaggeration. Consistency of implementations between browsers is better than its every been. There are still some vendor-prefixed properties to care about, especially if one is generous in one's support for older browsers, but that's another reason CSS tooling exists, to write only standard CSS and let the tool fill in the older variants.