The way the IFC specification is published follows a standardisation process (ISO, CEN):
Any technical documentation made available by buildingSMART International Limited is the copyrighted work of buildingSMART International Limited and is owned by the buildingSMART International Limited. It may be photocopied, used in software development, or translated into another computer language without prior written consent from buildingSMART International Limited provided that full attribution is given. Prior written consent is required if changes are made to the technical specification.
This is very different from what is usually thought of as what Open Source means. E.g. licenses such as GPL, LGPL, MIT, … give you particular rights to use and adapt and release new libraries or tools but also obligations. Try not to confuse the two.
I do agree that multiple open source projects are supporting the buildingSMART standards and they can complement each other really well, but but the majority of work that is performed with BIM technologies is not based on Open Source technology and the biggest financial support within buildingSMART from software is from commercial software vendors, who are not publishing their technologies as Open Source. They don’t have to - I’m not judging this.
Notable exceptions are a.o. XBIM, IfcOpenShell and BIMserver.org and your work on BlenderBIM. Yet, none of these projects has the financial means to be a buildingSMART member nor partake in the certification procedure. It would be interesting to see how the two worlds may strengthen each other more in the future.