Real world grasslands have been converted to some of the breadbaskets of the world. As portrayed, the only reasons that Mulgore isn't one giant farm are Blizzard's abysmal ignorance of agriculture and laziness in world building.
Jungles and swamps can be and have been reclaimed to become farmland.Feralas is a jungle. Jungles are notorious for producing very little food, not to mention the area is shared by both factions.
Tirisfal isn't shown as plagued, neither is Silverpine. Haunted by the undead, sure.Tirisfal and the Plaguelands were originally done in by the Scourge and whatever yields they produce are likely only edible to the Forsaken.
The Horde was given Azshara. Not them seizing it, it was GIVEN to them. They proceed to build goblin rocket raceways and keep whining they have no fertile land. The reality is that both they and Blizzard are unwilling to put in the work needed to make farmland.The Horde's lack of access to fertile land has been canon since Cataclysm, and I don't think there have been developments to change that since.