It's mostly meant to encourage you to not burn your Jungles too early. Jungles in AND are not as bad as Jungles in BTS; Swamps are the bad terrain feature that you have to put up with early on until you reach the technology necessary to remove them (Canals). Also, Jungle Camps do not improve with technologies, while other improvements get multiple stages of improvement. Furthermore, improvements that spread out their bonus over multiple yield types need to have a higher total yield compared to focused improvements (Farm, Mine, Cottage) to stay competitive. This is why Watermill has a bit more total yield than other improvements that are possible on riverside tiles.