But why the insistence on taxing land at it's full value while ignoring what's on the land? High taxes on just the land force me to sell and to improve the prop

But why the insistence on taxing land at it's full value while ignoring what's on the land? High taxes on just the land force me to sell and to improve the property. High taxes on just the house can force me to not improve. Neither seems ideal. What would be ideal to me is a good balance that rewards improvements without punishing those who don't want improvements. Especially if the owner lives there.