I can't speak for everyone else who makes YouTube videos, but I personally do both. I tour with a lot of pop artists, have an awesome career as a touring musician in LA (Just played to 10,000 people on NYE with a Disney Star in Hershey, PA) and also make YouTube videos when I am in town (and sometimes on the road).
YouTube allows me to express myself in ways that I can't on my usual gigs (like playing solo bass arrangements), and it allows me to connect with and help many bass players around the world.
I love playing live and would never give that up. After all, that's what got me into music. However, YouTube provides an equally rewarding experience and I've gotten emails from people telling me how my videos got them excited about picking up their bass again, or how it inspired them to start playing bass in the first place. This is highly rewarding and motivating to me, and knowing that there are people out there getting value out of my videos pushes me to keep making them and sharing my knowledge with this vast and awesome bass playing community