I think it comes down to old school thinking. Your competitors are providing extra content so you have to do it as well to keep up, so they stay at your site versus going somewhere else. They provide extra content on their sites to keep up with you and the cycle continues.

Pay sites have often, and still do, used boasting on the amount of content as a marketing tool. It's old and it still works. If I can say I have more videos than abc.com I'm hoping those people will come to me instead. It would probably only take a few sites doing it successfully to change the trend.