GNU generally requires that contributors assign their copyright to the FSF, which is what allows the FSF to litigate; they can't do this for software that they don't hold copyright to. The Software Freedom Conservancy, on the other hand, holds partial copyright to each of their many member projects' software and chooses to actively enforce the GNU GPL on that code quite frequently.

The SFC's ongoing case against Vizio in California[1] appears to show that violators of the GNU GPL can be sued by users, not just copyright holders. If this case sets precedent elsewhere in the USA and even in other countries, then organisations like the FSF and SFC will have far more options in what violations they choose to litigate. However, this also opens up the risk of 'GPL trolls' who might not be motivated by the same principles[2].

[1]: https://sfconservancy.org/press/vizio-coverage.html

[2]: https://www.fsf.org/licensing/enforcement-principles