Don’t blame the user as much as the phone. The fixed focal length means that if you actually want to get more than just a row of heads, you have to shoot close video vertically, and many phones put important buttons (like power) in a location that makes it easy to accidentally press them if you’re holding the phone firmly and sideways.
People also generally use their phones vertically, so it’s not too surprising that people will quickly shoot vertical video (and get the shot) rather than risk not getting the shot by worrying too much about things like “framing.”
