Why is traffic so slow?

Usually driving in freeroam with traffic, I always thought it was too slow. Even with no powerful cars, I always can pass any other car, even on the speedway. I thought this was because developing a good AI to make the traffic drive faster was difficult.
Then I recently discovered this possibility of defining a destination, then start a race to this destination with AI cars around. They drive extremely well at very high speed, I can't follow them! So it IS possible, having AI drive fast traffic is definitely possible.
So why is normal traffic so slow? Do you think the developerrs could make them faster?

 

Just in case someone is interested, I found a way to make the traffic much faster and it makes the game much more interesting and fun, in my opinion.
I did this with the Traffic App, I wish someone told me earlier...