Not just design work though, but also content and features work, which pulls in product development and sometimes, infrastructure people. Responsive/mobile design necessarily makes content disappear or moved around, which can substantially change the function of the site.

For example, in Adioso's implementation:

http://adioso.com/us/new-york-new-york-to-rome-it-next-month...

...resizing to the mobile width hides the "Lock Flight" buttons that are normally placed in the far right. So someone had to agree/argue that hiding those buttons was a worthwhile compromise (as opposed to stacking them on the bottom).