How about the fact that most successful businesses that have ever existed have charged for their products? Why is this data that can safely be ignored? It's the most proven business model out there.
Furthermore, it takes time to get good at anything. As you implied, making money from advertising is a skill, so how long does it take for you to get good enough at it to support yourself? Charging for things is simply more conservative. It's for people who don't want to risk going under while learning how to advertise.