Many reasons. Main one is because sitting directly on top of your fish is a bad idea, especially if you are fishing clear or shallow water. A longer rod helps you put distance between the fish and the boat. Another reason: Typically when vertical jigging brush, you are going to be in 12 fow or less. A long pole lets you set your line to the depth you are fishing, and you can pull the fish in without reeling. Just unhook the fish and drop right back down to the same depth without having to guess how deep you were fishing. If you get hung, just reel up to the jig and poke it loose. Cant do that with a short rod. Fishing brush deeper than 12' is the only reason I would consider using a shorter rod, as you lose most of the advantages of a long pole at those depths.
Also, I fish a lot of lily pads and Cypress trees. You need the reach when fishing visible structure as well. The last thing you want to do is bump the tree and get tangled in the limbs because you are trying to get too close, or pull right up into a patch of pads and spook all the fish. Very difficult to fish this type of structure with a rod shorter than 9-10 feet.