The difference between a CSS transition and a responsive image is, of course, how they degrade. If a CSS transition doesn’t work, who really cares? Your interface might be a little jumpy, but the experience as a whole won’t really suffer because your users will still be able to accomplish their goal and consume the content they need.
That really isn’t the case with images. How does a new image tag degrade? The img tag is so widely accepted that I couldn’t even find when the W3C recommended it as a standard, other than a small reference in the HTML 4.01 specification. Replacing or even expanding on the img tag would be like telling Frank Sinatra to wear a baseball cap instead of a fedora — you’ll get some pushback.