Earlier quoted context omitted.
It's solved in some [rather left leaning] countries. When you buy land you must put it to use, and if it's unused after X years, it's considered abandoned and it goes up for sale again. TBH, I think it's a solid model -- buying a finite resource like land merely with the intent to deprive others of it and make a profit in the long term is as unethical as it gets. Though completely legal is most places, of course.
With land, not using it is the equivalent to a privately funded conservation site. Its not necessarily a bad thing. Many capitalistic countries also have adverse possession laws, where if someone uses the land for X number of years, it becomes theirs.
This loophole is solved by making it that declaring the land as such status means the land is commited to that use for a minimum number of years that makes the conservation meaningful. Something like a decade minimum with a high bar for converting it for other usage afterwards. Farmers in the UK for instance already have some incentives to leave some fields alone.