It's not that designing these themes is hard. The process is not hard.
However, there are many things which do stand in the way of a hand coder's progress, and can seem more like php authoring than css and html design work. Especially for one who is set in their ways.
My first effort was back on B2 and all i could do was design my own page, and then stick the Loop into the page. I found the prospect of creating a "theme" totally daunting.
After several years of experience, i am comfortable with the language, and know my way about an index.php a little bit better.
So essentially, while it is not "hard" it is very much an enculturation process. Getting to know the language, and becoming (if not flexible enough then at least) accustomed to what's required for WP to function and not fall apart is really important for producing worthwhile results.
That said, even with all my experience, my favorite designs to date break in IE, or only work well under certain conditions.
I've taken to using the images in the new 1.5 default theme, and customizing the better part of my installs this way, then using CSS to customize further.
I'm pleased with this process, and it's visual variety and perceived complexity, but it's not as much fun as coding from scratch without language, limits, or unintegrated code to consider.