I suspect future earnings potential. A couple hundred acres of good farmland, if cared for, can be used to make the owner money from now till whenever: if they put a lot of work into it, mind you, not like it's just sitting there earning interest. Also, it's a limited edition thing: they aren't making more of it. Well, slowly anyway, geology takes a while.
Something else to consider - the land is likely to be mortgaged to to gills to pay for the really pricey equipment needed to farm these days. Farm equipment makes exotic cars look cheap in comparison (fun non-sequitor fact: saw more Lamborghini tractors on the road in rural Italy than sports cars).
Many of the same arguments on income vs. net worth apply to most small business owners. It takes a lot of $$ to get the infrastructure to run whatever enterprise they're running, and it's enough to support a few jobs. But, it's not particularly liquid infrastructure: cash out the land or the pizza oven or whatever, and no more jobs or production.