I've witnessed this become a fantasy trope since I was younger. I'm not sure why its blossomed the way it has, but the first time I noticed it was in the Forgotten Realms boxed set. There's a notice board in downtown Shadowvale where the locals place advertisements for need and adventure. Now, the Forgotten Realms is a fantasy setting and it postulates the widespread literacy of its inhabitants. However, this has unthinkingly developed into an almost automatic trope: everyone in fantasy settings is generally presumed to be literate. Why is this?
The most likely reason is that this is the way our world works and fantasy is inherently parasitic, where undifferentiated drawing on the real world that we experience every day. I don't know if it grew out of the Forgotten Realms use, but I've seen it now spreading across media. It made an appearance in Dragon Age, and has recently been added through a mod to Skyrim. I don't understand this (unless its just laziness in attempting to fashion a realistic regime, or lack of desire to do so), but I think I can take it as an opportunity to launch into a discussion on literacy in the Middle Ages and, in a broader sense, in fantasy as a whole.
Literacy was common amongst the upper class of the Roman Republic and Empire; indeed, writing and the written word was key to the ability of the Empire to persist over the time that it did. Administration was undertaken by an entire class of functionaries in the later empire, much of whose existence was taken up with magisterial duties that required the ability to read and write. However, the superstructure to support this kind of learning (primarily the rhetors and grammaticians who traveled from family to family teaching their children) collapsed and so did the learning.
The art of reading and writing was not one that was commonly known after the collapse of the Roman educational system. Male nobles commonly were not literate, though there's a caveat there that has to do with the administrative systems of Charlemagne's short-lived empire. The most common literates were priests and women.
Clerics often acted as secretarii to nobility, reading and writing for them. Indeed, this may be where the Song of Ice and Fire maester comes from. Almost all manors and keeps would posses at least one confessor or chaplain for their lord who may or may not have doubled as the village priest. This cleric generally wrote and read for the lord. It was also possible, as the middle ages wore on, to find secular clerics (canons, mostly) who would fulfill these tasks.
Bishops, monks, abbots, canons, deacons, these types of figures would commonly be trained in the Latin tradition. For everyone else, writing was unnecessary, extraneous, and something that would likely be scorned. Who has time to ruin their eyes in the dark rooms of a monastery or the glum grimness of an incense-perfumed church when they need to be training to fight and win on the field of battle? Yes, the wisdom of the ages lies in books, but books were rare and hard to come by.
This brings up another interesting side-effect of the generally illiterate culture (or perhaps a precipitator), and that is the lack of reading material. The most well-read men of the middle ages traveled from place to place or requested, in no uncertain terms, copies of books be sent to them. Books were fabulously expensive, requiring the killing of numerous animals to produce, as well as hours and hours of preparation for material and further laborious man-hours in copying out.
It is strange, then, to imagine medieval peasants wandering up to notice boards and leaving their pleas for adventurers. Adventurers, unless they were formerly priests, would be unlikely to have the knowledge to read such pleas (and the farmers would be unlike to have the knowledge to write them). Of course, this all changes as the middle ages wears on toward the pre-modern era, but most fantasy settings take place before (or without) the invention of the printing press.
Once again, I'm certain there will be angry folk who sneer and remark "This is fantasy, not history," but if your fantasy is no deeper than a bunch things you thought were cool all welded together without any thought, logic, or consideration behind them, perhaps you'd be better off remarking "this is trash, not history." So certainly, reject the literacy/illiteracy debate, but reject it in an informed manner. If people are literate, why are they literate? How? How does this affect the rest of society?
Or just do whatever you were going to do anyway, I suppose. It's not like I can stop you.