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.
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.
The only way I can make sense of this sentiment is that people feel like CSS is something to hack with, it is a toy, for hackers to play with, not a tool for professionals who spend years mastering the skill. I wonder why this is.