QI is a pillar of modern civilisation; this revelation is worse than finding out the Stig is human.
Sessions looks possible due to his smugness and a very high answer rate.
Carr? Yup, could be, similar reason. Carr also demonstrates the Woss effect - even obnoxius twats can be quite bearable in the right setting. I quite like Ross (on a good day) on his own radio show. When Carr is on QI he seems to briefly convert into a human being, with mostly inoffensive opinions, some of them quite funny.
Carr and Ross are both insecure show-offs, which make them pretty unbearable in certain scenarios.
Fortunately QI is so marvellous that things like cheating have no effect on its pleasure-giving properties