It's not a person. Unlike a person it has a tremendous "memory" of everything ever done its creators could get access to.

If I tell it what to do, I bias it towards doing those things and limit its ability to think of things I didn't think of myself, which is what I want in testing. In separate passes, sure a pass where I prescribe types and specific tests is effective. But I also want it to think of things I didn't, a prompt like "write excellent tests that don't break these rules..." is how you get that.