Whoever is giving the child access is responsible, not the manufacturer. If a parent gives their child a device capable of accessing the internet with no restrictions, that's on the parent.
Pornhub is manufacturing a product and making it available to the open market, just like Jack Daniels. Jack Daniels has no responsibility to ensure a bar is only providing access to legal patrons.
In your analogy, the bar would be equivalent to a internet cafe or public library that has PCs available to patrons. Those types of businesses should definitely use physical IDs to verify patrons are of age.
To make your analogy work for Pornhub, you'd also have to argue "why shouldn't Jack Daniels have to put age-verifying instant blood tests on their bottles in case a parent puts one in their unlocked liquor cabinet?"
Because then the same concerns arise -- why should Jack Daniels be given access to my blood just to manufacture an age-restricted product? What will they do with it? Will they secure the data appropriately? How do I know it won't be used to negatively impact my future because my health insurance company doesn't like that I drank a bottle of JD?