Why they don't use zero knowledge proof? Also question for the USA constitution experts, is this considered a violation of free speech? The article is not clear on this.
Zero knowledge proof is either trivially defeated by re-using the same credentials or doesn't have useful privacy guarantees. There really isn't an in-between here for something like age verification.
It's funny because the same "perfect is the enemy of good" argument is used both to criticize age verification in the first place (why bother if it isn't perfect) but then also to dismiss proprosals to implement it better (why bother if they don't perfectly fix the problem).
No. It's mostly that the proposed age verification schemes have fundamental problems that disqualify them from being considered "good" and none of the "better" implementations fix those problems at all.