If it's french wine, then import costs is the biggest...taxes and tariffs, but also transport costs and distribution in the US all taking a cut.

For wines in a top US region, you wind up paying a ton of money for the mortgage on the land, too.

I know lots of US states have egregious liquor taxes. Alabama is particularly bad: 58% shelf price + $.45/L

Part of that comes from having to pay tax on the barrel every year while it ages.

I don't think Alabama is aging a lot of wine relative to the US market as a whole. It's primarily a "sin" tax. One of the few taxes in the US that can gain popular support.

High taxes, transportation cost, and, (for explaining why Californian wine in California is more expensive than French wine in France), economies of scale and subsidies in France.

Frequently the wine seems hyper local - what I was there in a restaurant we often just ordered the house wine. If you talked to the waiters or owners it was frequently made locally and including by the owners grown in backyards of the hole in the wall places. I think it cuts out a lot of costs - and there was not a bad wine among them.

I always thought that "house wine" is just an euphemism for "whatever we could get our hands on" - a workaround for the restaurant so that they do not have to list a specific brand on the menu.

That is likely true from time to time, but most restaurants seems to keep it fairly consistent as far as I can tell. I think it’s more often a choice of taste on the part of the restaurant for a less expensive but decent wine.