Well yes but that's also the case with property taxes. A reasonable figure based on current market values seems simpler to calculate for unimproved land than it does for property tax, which includes the value of the unimproved land as well as everything else built or changed. And because it's the value of the _unimproved_ land it's based on, the value would be relatively stable over time as well as not varying much between one area and similar areas nearby, whereas property taxes are based on values that can change dramatically over time from one place to the next, which can lead to mis-valuations and general stress from constant changes.