It's all about advertising. On a web page, which is cheap to create and cheap to host, the only ad you can really get is a pop-up or similar, and those don't pay very much. On the video, which is expensive to create and expensive to host, you can have 30 or 45 seconds of video ads, which pay a lot more.

It has nothing to do with the expense and everything to do with the fact that most people do not enjoy reading (any more, at least).