They inheritted it and have been lazy, they bought it for an investment, they bought it because they might want to build their retirement home on it, etc. Plenty of reasons why.

I know people who have bought the land they want to retire on now. (if they will or not is a different question, but that is the current plan). I know people who own hunting land that they visit one weekend a year. There are people who own land to lease to a local farmer - many farmers have too much money tied up in land and want to lease some of it from someone else to spread the risks.

If the land is expensive you wouldn't let it sit, but there is a lot of land that isn't very valuable that you can just own if you feel like it.

I grew up on a big property and when my parents moved, they divided the land and sold the part with the house with the idea that they might build a house on the empty land upon retirement.

I'm guessing at this point that they're not going to do that, so at some point I'll probably inherit some empty land.

Optionality. There is also a lot of cheap land in the US, so it often isn't a large investment.

See I can understand this. e.g. a family asset for children, a property to build a retirement home on, a hunting property, or simply a real investment.

Buy low, sell high.