Because people don’t read nearly as much as they watch videos. Most YouTube videos are people talking to the camera or over images/sequences that could just as easily be removed. You can (potentially) reach a much larger audience on YouTube than you can via your blog/website. Increasing reach and (thereby) earning (some/more) money is far more easily done by using video than text.
After all, it’s the reason why films and TV shows are generally more popular (and therefore potentially much more profitable) than books.
Edit: And while I was writing the above @sillhouette posted a relevant video. Oh, the irony. ;-)