In Interesting Times, Arch-Chancellor Ridcully says that Rincewind is not technically a wizard because he failed at the University and has never performed any "great services to magic". Rincewind responds that he has saved the world a few times, to which Ridcully responds, "were there any witnesses?" Rincewind was apparently stumped at that point. The problem is that in The Light Fantastic, Rincewind reads the Eight Spells from the Octavo and saves the Disc, with the roof of the Tower of Art, which was crowded with witnesses at the time. Why doesn't anyone remember this?
The witnesses present at the time of the reading of the 8 spells are the townsfolk of Ankh-Morpork who were in the process of lynching all the wizards, the librarian, the luggage. None of whom are particularly viable witnesses, 'ook' has many meanings but it's not that useful for complex concepts.
Or if we consider the Bel-Shamharoth (The Sender of Eight), the witnesses were Twoflower, the luggage, and Hrun the Barbarian. At least two of whom were too wrapped up in their own business to notice.
Then we have the incident with the sourcerer (Coin). After which any surviving wizards made a point of claiming not to have been present at the time. Which means that even though many of the senior wizards do know that Rincewind saved them all, they're never going to admit it, because it would mean admitting siding with the sourcerer and hence being on the wrong side.
Hence, no witnesses to Rincewind saving the world.
Because wizards have incredibly selective memory, and the librarian is one of the only people still alive after competition and Coin ravaged a few decades ago when Rincewind was busy running away in Colour of Magic and The Light Fantastic.