94% of people who play horizontal videos on mobile phones do not rotate their phones (out of simple laziness) and see a reduced image, surrounded by black bars.
It is now possible to display videos in full screen, regardless of device orientation. Streamlike offers a display mode that allows you to remove black bars when a horizontal video is played back on a phone held vertically. The image is zoomed and the subject remains visible in the center in more than 80% of cases. The loss of resolution is imperceptible on a small screen. Judge for yourself on this example of a video shot horizontally with a phone:
Another example with this video optimized for both orientations. If you play it on a phone, switch to full screen and flip it over during playback.