I've helped convince a few designer friends and colleagues to get into HTML / CSS as they progressed into their careers.
It always seemed to most of them like a daunting leap from the creative application of graphic editors to the very manual text-based tools (and sub-par graphical tools) for rendering designs in a browser. Most would refer to HTML/CSS as "Programming". The majority of their experience and education had been with graphical tools, so the idea of tweaking layout and composition with text was completely foreign.
Most [art] schooling up until about a decade ago was largely based on print design, which means photoshop and illustrator. There may have been a couple classes here and there about building a website, but the vast majority of college level classes for graphic design were all about print and various other analog mediums.
This is actually what eventually led me to drop out of graphic design school in 1998 because it was so far behind the times in terms of web design, which is where I wanted to be.
So with the reluctance of the most talented designers in the industry to learn the medium, it was left to the developers to cover the gap - generally, quite poorly. I've been through plenty of back-and-forth annoyances with print designers - in the very distant past - while they would ask me to tweak pixels that I could hardly bring myself to care about when I had so many other priorities on the project.
For years I'd require any designers I'd worked with to hire "frontend developers" to handle the pixel tweaking and html/css generation, which would eventually start counting against them as the design industry started catching up with the medium.