It could be that Google Maps is saving everyone time by doing this. The Braess Paradox is really interesting, and this could be an attempt to prevent it: http://vcp.med.harvard.edu/braess-paradox.html

I would imagine introducing cars to a road itself causes a braess paradox.

Introducing cars does cause more traffic.

However the interesting thing about braess paradox is that even if you keep the number of cars constant, introducing a new road can make travel times worse.

It usually suggests three routes to me, each with a time estimate, and the highlighted one doesn't always have the least expected time. I figured it might be respecting my other preferences, but it could equally be trying to balance traffic across the whole network. Though this also happens when cycling, which has a minimal impact on congestion.