How would you know that?
Exposing people to random hazards and seeing if they are harmed has its downsides. That possibly powered people would run away is a consideration that would quickly affect even those insane enough to not realize it would happen. And of course, there are scads of powers that might be hidden like that.
And they do not have a device to test you. It would be a hard one to make, even if they had time -- I'm not sure about the time, though the powers have been around long enough for things to stabilize a bit.
And no one has a power to figure out other powers. And the powers are distinctly random. . . .
All this and all the main characters have overt powers they are aware of -- all background details here.