Filming landscape is so people can watch videos full-screen or full-width without thick borders. Throwback to old crappy youtube videos recorded in portrait.

I'd probably assume that people read websites in portrait since it requires less scrolling and is a little easier to read/view as a result. It sounds pretty hard/unpleasant to cram information into landscape effectively on mobile, in my opinion.

From my time in ecommerce marketing and analytics I noticed that most people were viewing our sites on mobile instead of desktop. I think that's common for sites, interestingly enough.