Maybe there's a really low literacy rate in the OP Universe.
That is possible. While there is the World Government, there are different governments for the individual nations of different levels of technology and different cultures. Some islands are not part of the organization and some islands do not have a government despite having some people on it. Orange Town's mayor said that they settled that area after former settlement was destroyed by pirates…nothing was mentioned about any government except the mayor there, no 'asking the government/king/ect. for help or if they can move their town.
With such a variation of government, its easy to assume that literacy isn't as spread as it is in the real world.
Heck, I see some aristocrats like the Goa King and his ilk (you know, the king that had the Grey Terminal burned down - along with the people in it- and the nobles that were not bothered by it, in fact they called the people there 'trash') would want to keep the lower class illiterate. Similar to how some of the upper class in Victorian England argued against teaching the lower class how to read since "it could lead to them being unhappy with their lot", if they could read and could lead them to organizing riots if they could write to communicate with each other. (now not all believed that drivel but some did)