I think it has to do with compatibility, one size fits all. Also I don't think social media was gearing up for this but rather designers and artists who took to social media to market themselves and that changed the whole perspective. These apps were and are still targeting a different demographic and were designed for entirely different reasons. I don't think apps like instagram even realised people were gonna start posting design work there, but they do now and have made small changes to accomodate that part of the group. But their majority crowd likely are the ones who just wanna post a pic of their fav food or some random shit like that. So social media apps weren't designed for designers to post work but it has become the norm for designers to use social media to market their work. If you want a free ranging aspect ratio or other customisable features, then like most designers do you simply have to have your own website.