[giantg2]

"underutilizing their land but I think most places are using their land for the highest value thing it is zoned for."

In urban or higher density suburban areas, I think you're mostly right. But in lower density areas, a lot of the land is multi purpose or has few restrictions. So technically, you could build apartments or homes on land that is used as agriculture. However, there might not be demand for it in that area. Essentially, the tax-by-zoning works well in the situations where zoning is restrictive and demand is high. But in low density areas where the actual use is not well defined with a single zoning use, the value would be potentially arbitrary or inaccurate (making property tax a better measure of value).