Advertising. It is much harder to inject custom, "targeted", ads when a user simply downloads the video (the ad's would have to be injected into the video, dur

Advertising. It is much harder to inject custom, "targeted", ads when a user simply downloads the video (the ad's would have to be injected into the video, during the download, to allow for targeted ads). Meanwhile, playing in the browser allows the surrounding space not used by the video to be filled with ever changing targeted ads.