In addition to what buzzy_hacker has written, in a normal FOSS project A I can ask maintainer X to include a feature from a compatible FOSS project B written by

In addition to what buzzy_hacker has written, in a normal FOSS project A I can ask maintainer X to include a feature from a compatible FOSS project B written by programmer Y. The maintainer can do it themselves, or I can adapt the code myself and submit a patch, referencing the original authors. That's how FOSS is supposed to work. In a CLA-restricted project, there's only one entitity that can contribute copyleft code. Everyone else must donate the code to them, and they forbid themselves from using other people's copyleft code, because they can't relicense it.