At least a few years ago, thinner media catalog with deeper viewership ends up being better financially. Some friends and I looked at what it'd take to put up a competing service and there's a lot of cost in all the CDN traffic, when a video gets dropped off from a regional distribution node then has to be pulled again from storage. I'm sure YouTube don't need to worry about it so much, but a higher eyeball-per-MB means more ad revenue per MB and lower infrastructure costs, I think.