Furthermore, I have better things to do with my time than spend time mucking around with configuration files and theme settings. Back in high school I had the time and inclination, and in fact I did so; now that I run a business, I'd rather spend that time doing actual development. Not to mention the moms and grandmas of the world who don't even know what a theme is. I, and they, need a UI that just works out of the box.
And finally, just about every computer that is responsible for my internet access runs on top of software written by large corporations who have hired people to develop for open source bodies. Let's not forget that it's for-profit companies with full-time programmers like Red Hat, Canonical, IBM, Intel, etc. that are more than partly responsible for the relatively wide enterprise (and desktop!) adoption of Linux today.
I'm not out to bash Gnome, Linux, or open source; I use Ubuntu on my work laptop every day. I criticize out of the desire to see the open source movement develop something better, and that means not patting everyone on the back just for "working hard." It's precisely that kind of "gold star for effort" attitude that lets crappy software exist.