If they didn't have to retake the test they would have 0 stress because they'd be done, so you can't really claim there's less stress. Also many students likely already crammed enough knowledge into their heads for when they took the original test, and are now forced to retain all of this for multiple weeks, i.e they have to put in a lot more effort studying for little to no improvement in score (yes students cram for tests and don't retain information, that's just how testing works)