I’m unsure whether YouTube was just attempting to force the world to conform to home-media aspect ratio, or whether there was some sort of compelling reason behind it, but for whatever reason videos shot vertically always were displayed like they were filmed by a two year old using an early webcam.
That’s over now with the newest version of YouTube, which you’ll either be prompted to update or you can grab a copy from APKmirror if it’s not showing for you.
Does this matter? A little. It means videos shot on a cell phone in portrait mode will display properly on a cell phone in portrait mode, and will require home audiences to tilt their head to the side or reorient the TV to see the video properly.
I see only chiropractors truly benefiting from this. Chiropractors and hipsters.