Autoplaying (without the possibility of sound) is a feature, It's a huge aesthetic boon to the web. GIF is an entirely different conceptual flavor to video. It's good precisely because people almost never fuck with the fact that it autoplays.
That GIFs autoplay is unequivocally a bug. Just a well-entrenched one that would take a lot of effort to fix, and no one capable of fixing it seems to quite think it’s worth it.
There are multiple reasons I might not want autoplay: bandwidth or performance concerns, accessibility problems from motion, or even just the simple “if you autoplay, then when I reach it it won’t be at the start of the video any more”.