Because it's interesting and "because we can". It's a challenge without any other motivation besides curiosity.

But lets say some ransomware outfit discovers farmers as their niche, because the security barrier is relatively low and it's a time sensitive business. Your crops are ready to harvest, but your equipment is not starting until you pay the ransom? What can you do then? Waiting and letting the crops rot is not an option, renting hardware from others can be difficult/expensive/impossible, so the most would pay the ransom. I haven't heard of attacks targeting farmers/farm equipment in particular, but it could be a real problem in the future.