We would want to do that for the same reasons that people write textbooks. Mathematica was not designed for building applications like Java or C++, but as a tool for research and learning. To that end, Mathematica is well designed and the documentation is exceptionally curated, just like, for example, a physics textbook, which also encodes public knowledge in a private (copyrighted) manner.

The mathematicians and software engineers at Wolfram Research should be compensated for their efforts, just like the authors and publishers of textbooks. And I'd much rather pay for my textbooks than see them filled with ads.

As a lover of mathematics I'm happy to see people writing and thinking about the ideas in this post. It's a noble pursuit.