Assuming that content is shot in landscape, I see a lot of people post screenshots on here on mobile devices asking for IDs which have the black bars at the top and bottom to which suggest to me that if they realized they could turn their phone the other way to have a better viewing experience then they would also do this when it comes to taking a screenshot as well.
But anyway, I dunno. Maybe it's a TikTok/Instagram era tradition for them all to be shot in portrait, or whatever - and not give it a second thought.