Indeed, IDEs are still oriented towards writing code, not reading it. I'm yet to find a good tool that would help with exploring and familiarizing yourself with a large codebase. Soucetrail comes the closest, but it has like 5% of the features I'd like to see in it.

(My dream code exploring tool would be a cross between an IDE and a tablet&pen friendly PDF reader. I should be able to navigate through files using all the semantics-aware IDE goodies, the tool should also understand version control, but it should also be able to generate and manipulate structural graphs (as in, boxes and arrows), as well as let me highlight, draw and freehand over the code.)