Because Chrome still doesn't sync your "search engine" settings. So maintaining a massive customized search engine across the multiple computers I use would be a pain. If this was a native feature it would work so long as I logged in to Google. Of course, if they fix chrome I'll be happy too.
Create something like:
http://www.google.com/search?q=google+-google.com&ie=utf...
I suspect you could hack together a quick greasemonkey script to automatically inject your negative site list when the URL is generated