Why don't people in Seattle have to separate their recycling before it gets picked up?

So a couple years ago Seattle citizens used to have to separate their glass bottles from their other recyclables. In Japan, they separate all of their recycling into different bins such as "glass only", "paper only", and "plastic" before they reach the recycling center. Why don't people in Seattle have to separate their recycling, like Japanese people do, before it reaches the recycling center?Why don't people in Seattle have to separate their recycling before it gets picked up?
Different setups for each recycling facility. In my small town we have single-stream recycling too. Sometimes the separation can be done mechanically; sometimes it is done by hand. Most people can't be trusted to sort correctly anyway. Can you tell the difference between HDPE and LDPE without looking at the recycling mark?