It is very difficult to explain extreme tiredness to people who don't fly. If you haven't experienced something yourself you cannot have an understanding of it.
Even Niki Lauda's experience of it was not long term and haphazard. Sure, if you turn up to do a long back of the clock flight once every month it is not a problem, but doing at least 3 or 4 every week for say 5 years has a very different effect.