Because their neighbours have sold their houses recently. So using similar sales as a baseline and then factoring in a few differences between those properties

Because their neighbours have sold their houses recently. So using similar sales as a baseline and then factoring in a few differences between those properties and the one that is being assessed is much easier than doing that and then subtracting the built-up value. Basically all of your data points are the built-up prices. So it is going to be easier to estimate the built-up value than the raw property value. Maybe not much easier, but definitely not harder.