I don't know, but the answer could be that a lot of the land is being purchased solely as a safe investment with a very small return and the expectation of an appreciation in value (i.e., the gain when they ultimately sell the land). Perhaps most people aren't buying the farmland because they want to farm it themselves. For the non-farming land buyers, the rents just give them a very small, and relatively safe, return.
In a weird way, it seems slightly similar to buying a professional sports team. The investment value comes when you sell the team - not from annual revenues and profits (which sometimes are negative).