Because land belongs to society at large. The entire idea that someone can own land is nonsensical. Private ownership exists to encourage people to create stuff but land can not be created. When land value increases it is not because of anything the landowner did, but because of what society did. The gains should go to society.

Its also more effective at removing speculation the higher it is. LVT is one of the few taxes with no deadweight loss, so even if it wasnt the most morally acceptable tax it still is the most economically beneficial one so we should try to collect as much as possible through it.