Yes, they do that, or at least they claim to. Out where I live, they can probably come close, because there is still some vacant land out here. When it sells, you get an idea of what nearby land would sell for if it were vacant.

But here's a building across the street from Central Park in Manhattan. What's the land price? When was the last time that a vacant lot sold in Manhattan?

I mean, I guess you can go by the cost of the last place that sold with a building on it, and the buyer tore down the building. (Subtracting out the cost of the teardown.) When was the last time that happened across the street from Central Park?

So in the places that the land is already heavily used, this is not all that easy to do.