A designer spend time building different concepts of designs, different colours and shapes. Designing in a browser takes so much more time than just using a tool like Sketch or Photoshop with a few clicks, and would be such a waste of time to try different things. I, myself as a developer, use Photoshop to design websites and mobile applications because I want to visualize what I’m going to build first, play with the design before I spend much more time on the code to build something.
And for someone who’s not an expert in HTML and CSS, the code quality would definitely not be as good as from a developer. I definitely prefer to build everything on my own based on a design, as I know the code I will produce will be perfectly optimized rather than getting some wonky code that might not work in all browsers and requires to redo everything anyway.