Because this is only checking the individual function, not the entire systems state. If I have a function that adds two ints I could test it with [TestCase([2,

Because this is only checking the individual function, not the entire systems state. If I have a function that adds two ints I could test it with [TestCase([2, 2], 4)] and this would pass but if it also changed the value of an object there could be an issue a unit test (and indeed integration/e2e test) wouldn't catch.