There's an abundance of materials online, the issue is that they're not collated or aggregated in such a way that a technically uninspired teacher could effortlessly utilise them.

This is a problem that I and many others are working towards addressing.

"Textbooks" however, being books written to directly accompany a course, are probably not going to constitute the prevailing paradigm online - they are static and structured in a way that traditional printing required but modern technology finds burdensome.