But it seems humans only tend to acquire and demonstrate knowledge for some sort of self gain. These tests would then just tell you the general level of apathy of the students and do not tell you how much they know.

If I wanted to know how well the average person on the street can clean a toilet and my test involved asking a bunch of people for a demonstration without pay nor feedback of any kind I don't suspect I would find a lot of clean toilets. I would instead have learned that people don't like cleaning toilets and gained no insight into their ability to do so.