Seriously, there's some attitude in open source circles that looks down on using the law to achieve compliance. Why bother putting up such a carefully thought-out legal document, and then whining on a blog when it's broken, instead of using it for its intended purpose -- a court of law?
Also, due to the ill view of patents on this forum, I imagine people here are simply put off from using the law and thus opt for the dopamine hit.
I'm not talking about HackerNews in particular here. There's a deep divide in the Linux kernel community, for instance, about GPL enforcement. Some people, often represented by the Software Freedom Conservancy, believe in "speak softly but carry a big stick": i.e., try by negotiation to get violators into compliance, but sue when negotiation fails. Other people (including, IIRC people like Greg KH) have at some point openly stated that they would never consider a lawsuit an appropriate course of action.
I'm not sure where the Leela Zero and Stockfish people stand on this; I just sort of assumed from the tone of the article that they were in the "never sue" camp. Maybe they're in the "I didn't realize we could do that" camp, in which case I'd strongly recommend they get in touch with SFC.
Because
* legal action is expensive, time-consuming, and may be simply impossible (or equivalently impracticable) depending on where you, or the other parties, reside/do business; and
* legal documents do not in actuality exist to enable someone's courtroom drama fantasies. In anticipation of civil disagreement, competent lawyers will advise that adversarial proceedings are a last resort, not a first measure, and draft documents accordingly.