What is the physical craving?
The desire for more, which converts into obtaining and consuming more.
Why 'craving'?
Because it's not rational. You're not thinking through why you want the third bottle one of wine. It's just wanted, strongly. Rationalisation comes in to justify it, but rationalisation is not reasoning.
It's also called a craving because it's overpowering. To the extent that reason kicks in, reason fails to keep us in check. We act against our own best interests.
It's a strange sort of craving. An ordinary craving is quenched by satisfying the craving. The craving they talk about in the book Alcoholics Anonymous is amplified by satisfying it, not quenched. It's like a mosquito bite: if you succumb to the desire to scratch the itch, the itch is amplified by the scratching. You've made it worse.
Why do we say it's physical? It's not felt physically per se; it's not evidently located any particular place in the body; it's obviously mediated by the physical brain (which clocks the signal and converts it into a plan); but it is kicked off by a physical act: the ingestion of alcohol. This is most evident where the first drink is accidental or casual. As soon as it hits the system, the system reacts, and, Boom!, the craving kicks in.
How else do we know it's physical?
If it's not physical, maybe it's mental.
Does that work as an explanation?
Is it thought through? No.
Is it rational? No.
Is it because we're stupid? No.
Psychotic? No.
Unhappy? Not always, so no.
Influenced by others? Ever drink too much alone? Yes? So no.
Circumstances? Ever overshoot in happy circumstances? Yes? So no.
Literally nothing else accounts for every instance of overshooting.
We drink the way we drink because that's the way we drink.
Full stop.