Of the many reasons, one is that they are highly effective at killing entreched infantry, which is the majority of the entire frontline. Artillary rounds that explode on impact or fragment in the air are safer for whoever inherits the land, but it take many more times the amount of those rounds to accomplish the same saturation as cluster munitions. This is a sad fact.
You're right though, if Russian farmers get the land they will be bothered by both sides cluster bomblets, probably killed. Ukrainian farmers might be happy to have the land at all.