Music Videos and "Hey-Cool-lets do fun videos with my cam" videos are two very very different thing.

Music Videos got the commercial feeling already in the name and people know that. But how can you monetize millions of stupid webcam videos ?

I know there are some exceptions.. It would be the same if you would say that MP3Music.com is hard to monetize just because UploadMusic.com (just an example) got already millions of MP3's uploaded by amateur users. Professional vs Amateur. Quality vs Quantity.
I hope you guys get the idea.