I actually think I know why this is, for Simcity 4, at least. The original SC4 traffic model, Simulator Z, spent more effort trying to find alternate transit modes (i.e., mass transit) if private road congestion was high. However, it was also very compute-intensive and wouldn't run decently on the lowest-end Intel PCs at the time. Maxis was concerned about backward compatibility, and so the game was shipped with a simplified traffic model that spent less time considering alternate transit.
If you install the Network Addon Mod (NAM) it includes, among other things, a new traffic model that's closer to the original design.
If you install the Network Addon Mod (NAM) it includes, among other things, a new traffic model that's closer to the original design.