Simple attestation is very useful for the case where a parent gives a child access to a computer and wants that computer to block porn. That's the use case everyone is clamoring for, and asking the root user "how old is this user?" solves it in a simple, open, privacy-preserving way. Everybody wins, except the teenager who wants to watch porn. If this were not legally mandated, everyone would support it as a useful feature, but since it is legally mandated, we have to get angry about it.

This has got very little to do with children - that is just the excuse that sounds good. "Think of the children" is a rhetorical tactic that anyone who wants to get unfettered access to your data rolls out whenever they can. It is a tactic that unreasonable people use to influence reasonable people, because it is so difficult for a reasonable person to argue against without coming across as uncaring and/or bigoted.