Large concerts rarely ID and are almost always all ages in the US. (Source: I go to 30+ a year, don’t think I’ve seen one ID.) Gate agent staff would have to be significantly increased.
It’s not unfeasible by any means, it just would cost money, and promoters like the scalpers. Most of the failures mentioned in this article involve a scalper buying tickets that would otherwise have gone unsold, and even for a show that would sell out eventually, LN gets the cash earlier and then maybe even gets a cut of online resale.