The problem is not that it's hard to cheat (it's easy), the problem is it makes you officially a liar and liable for "illegal app use".
It might not be a problem for you, but some underage kid, who lied about their age, gets addicted to a game with in-game purchases and gets into financial trouble now has no recourse against the company who made the addicting game.
There's no liability in the law for a child who uses an over-18-signaled account and accesses over-18 content, nor for a parent who gave that account to the child. It's all the parent's decision if the child should be restricted or not.
As noted in my other reply to you as well, this is false.
https://leginfo.legislature.ca.gov/faces/billNavClient.xhtml...
Does this imagined underage kid that right now lies about his age in UI-form have a case because it wasn't on OS level?
I genuinely don't know, and it's hard to see what's the differences between those two cases are.
I'm not a lawyer, but it's clear that this changes the narrative. If some technical restriction is in place (OS level age statement with apps who enforce it) and the kid circumvents that, it's easy for a company to claim that they did their part and all blame is on the kid. Without that, it's trickier for the company who intentionally created some addictive product to prove that they did enough to protect the kid.