The UX studies are taking into account the excess clutter when they aren't necessary. And that is a real problem that I have seen with maps apps that try to display them all of the time.

The problem is that it is really hard to automatically determine what is the most useful thing for the user. Right now, I think the balance is a little too tilted towards not showing them.

I think that one of the evidences of this is how many extreme cases can be shown where they clearly should be shown but they are not. Its much harder to find cases where the algorithm tilts in the other direction by mistake.