Ultimately, it all stems from two things - for one, it's illegal to emit radio waves without a special permit. And secondly, it's also extremely hard to process radio signals at the kind of rates we expect today.

Together, these facts make it so that (competitive) wireless modems require organized businesses to create, and organized businesses don't want to share their code with competitors. A foundation dedicated to creating open hardware and software for a competitive wireless modem would face giant hurdles both in regulatory terms, and in hiring people who can actually work on this extremely difficult technical challenge.

Also, building an open source software for controlling wireless modems that complies with the law is probably not fully possible. Per law, to sell a wireless device, you as the manufacturer are responsible for taking reasonable precautions against users misusing it to emit in reserved bands, or to not respect military device priority in the allowed bands. If every user is extended the rights and documentation for modifying the software as they see fit, you're clearly not taking reasonable precautions to prevent them from breaking the law.