The thing about asset variety is it puts way more pressure on streaming and memory management systems. If your environment consists of thousands of instances of a library of a dozen or so bushes, trees and rocks, the game only has to stream one of each once and repeat them. I wouldnt be surprised if most (if not all) of Rider's vegetation assets and terrain base textures are resident in memory at all times.
The hard thing about modern cities is they are filled with complex colorful billboards, store fronts, cars, delivery trucks, extravagant colours (pedestrians). While natural environments let devs get away with waaaay more repetition.
A city world as well realized as SM2's is almost certainly streaming way more new assets in between regions than Riders.