Not everyone has the ability to give for free.

Giving freely doesn't mean authors can't be financially supported. The Bible encourages financially supporting those in ministry. Some authors may even find they receive more funding than if they were to charge for a resource, as it ends up reaching more people who want to support the work.

Imagine a world where all Christian resources are given for free, and the exact same amount of people who pay for resources donate the same amount instead. In other words, a world where the exact same funding exists, but it is given voluntarily rather than by legal requirement.

In such a world, everyone would have access to the resources rather than just paying customers. Some aspects would cost more, such as to print physical books, but most resources are distributed digitally these days anyway at virtually no cost.

The barrier then is not funding but the hearts of Christians, to both give freely and freely support ministries financially.