Because kids have never lied about their age on the internet.
And if Itch started requiring age verification like through ID, credit card, etc.... well, no. That's pretty horrible, not to mention extremely resource intensive for itch themselves. The internet is what it is. If kids are accessing adult content then that's on the parents/guardians for not implementing proper parental control over their children's devices. Website admins are not a substitute for good parenting. There's an opt-in that must be enabled to see NSFW content, that's as far as itch really needs to go.
NSFW artists can get in serious legal trouble if minors get ahold of their work
Uh. No. Unless it's a law in some authoritarian nation, then no. This isn't even remotely true for the majority of the world.