Exactly, ARE. Not were. Who came first you think?
First we had viral videos and a lot of randomness, in 2007 I believe YouTube enabled Ads for the first time, allowing content creators to feed the website with regular content. This was the initial spark that even allowed the website to grow, as having no professional uploaders would have meant stagnation.
Only in 2012 with Gangnam Style, YouTube music videos started hitting absurdly high views. Now even lame songs such as Decpashito can be #1 viewed videos. I blame algorithms.
Musicians followed to a website with loads of traffic. Many songs were uploaded not by the creators anyway, and many musicians had a problem with their songs being ripped off YouTube. In 2005, the most viewed Video was Tupac's "Life goes on", uploaded by some random dude called fhearnoiz. I think Linkin Park were the first to actually land most viewed videos on their own channel. Not sure about that, though.
ONLY after VEVO became a thing, music songs were #1 every year.