Good point! I distinctly recall Imgur being started because it was one of those "this used to be something hard, but now Moore's law has made it easy" services. Imgur could afford to allow hotlinking because actually serving images had become a trivial cost. Because no other image-host had capitalized on this, Imgur cleaned up—for a while.

Once people started shifting to sharing content mostly as really-inefficiently-compressed ten-second videos (which are still pretty large when efficiently recompressed), Imgur's original business model was dead in the water. Maybe one day gifs will be as cheap to serve as images are now—but it won't be any time in the next few years.