The Web is a Different Medium
Not many artists would put up with working on a canvas that can change size and shape at any point.
I recently started to work with my girlfriend to develop her online portfolio to showcase her animation and 3D work. Right from the beginning she told me that she would handle all of the design work. As a creative person she could easily handle making some attractive.
What she has yet to realize though is how different the web is from any other type of art medium. There is not just one way to view the web which produces variable and variety into the art. Connection speed. Resolution. Browser. Monitor settings. All of these play a role in how the end user sees the resulting website.
This is just one more example of how difficult and unique the web design industry is. While anyone can write HTML and produce content, not everyone can make it look good.