This may be the case. While it is a standard practice in print design and graphic design that designers learn and use some tools to materialize these design into somewhat of a final product (tools like indesign, illustrator, etc.) it is not a common practice among web designers. A web designer is more likely to create a mock-up in a tool like figma, which a web developer than materialized into HTML, CSS and JavaScript. Of course there is an overlap, where a designer actually writes their own frontend or a web developer does their own design, but most of the industry has a separation there which is not present many other creative industries.
Perhaps there is a lack of understanding of the interaction between a web-developer and a web-designer which creates this unique negative perception of CSS as a tool. A back end developer might simply not understand it.
It lives in this weird in between land where it's not really a programming language but it's complex enough to be hard to use by designers who are not really into programming.
I agree with you that mastering frontend requires a lot of time and practice, especially because it's a constantly moving target with new features, new tools and browsers that are constantly evolving.
I personally like it, I love the flexibility and I love that it's constantly changing and I have new tools I can use. It's also funny how there seems to be always something missing.