Some applications made by commercial companies employ a paid/community model for their software, meaning that you can use the open-source "community" version with less features, or you can purchase a license to use the "paid" version, which is based on the community version and includes more features. (Think IntelliJ IDEA Community vs IntelliJ IDEA Ultimate)
I am wondering that if the source code for the community version is freely available and licensed, why don't users just fork the community version and implement the "ultimate" features on their own?