I think you inspired a pretty solid discussion, so I'm not sure what else I could add, but then this jumped out at me:

PS. Come on, its 2017, who still records videos vertically?

Vertical videos annoy me too, but they are formatted perfectly for Snapchat and Instagram stories, and YouTube just made an update following the vertical video trend: https://www.cnet.com/news/youtube-gets-a-redesign-new-logo-vertical-videos/

So, my guess regarding your question is that they're marketing towards the IG/Snapchat (and now YouTube) crowd.