Why can’t you use it? Because your web designer isn’t designing for the possibility that people use a phone to access the Web, but it’s not 2004 any more and they’re living in the past.
You showed your colours when you assumed that portrait video is of lower quality. It’s only of lower quality if you’ve padded it out and are watching it on a landscape screen!
Technology to detect whether your webpage is being viewed landscape has existed for a long time, and takes very simple calculations indeed or just a splash or two of css to maximise the video size for whatever screen it’s being viewed on. It’s design laziness and wasted bandwidth to put the silly blurry bars or even black bars down the side of the video. But don’t force landscape on everyone. Smart phones aren’t new and they aren’t going away.
I suspect that the majority of people who spend even a tiny bit more than half of their recreational screen time looking at a fixed landscape screen are well over thirty.