It may have to do with the setting- HP takes place on a fictionalized Earth, in nearly modern times, while ASoiF/GoT uses an entirely original setting. Having Harry Potter acquire a copy of his own canon doesn't require quite as much explanation as Tyrion walking into a Barnes & Noble or a book falling through an interdimensional rift and landing at Sansa's feet.

...Or I may be attempting to apply too much logic here and it could just be a side effect of different fandom demographics.

Anyone familiar with other high fantasy fandoms (LoTR, Wheel of Time, ElfQuest, etc), do you tend to see any fics with that sort of premise?