It sounds like you have truly learned this, but let's check. That's the secret. They think of it as canny loss-leader marketing. Just like proprietary software. It also cheats both the users and the authors of most of the benefits of free software.
Free software reduces the transaction costs of collaboration.
The more experienced users on the net help out the less experienced users to fix problems, including real bugs and missing features. They have fun teaching and learning about what's inside. For example, porting it to another platform. Or making it interoperate with something. Or adding a small feature.