This is a very good point. Ideally, one should be able to explicitly override personalization-related input as much as possible, so that one can freely explore

This is a very good point. Ideally, one should be able to explicitly override personalization-related input as much as possible, so that one can freely explore what kind of results other groups of people get. I suspect that Google are reluctant to provide such an interface as it might make it easier to game the system.