Souplex said:
Because scythes aren't weapons. They're farming tools. Only an idiot would fight with a scythe.
http://www.escapistmagazine.com/forums/read/18.70517-Scythes#694172
The real question is why do RPGs neglect polearms and axes? Historically they were the most commonly used melee weapons. Swords were the equivalent of an officers pistol.
According to the History Channel, swords were the elite weapon used by officers and those of noble birth until about the Middle Ages, when suddenly they became a more common weapon due to the development of new methods of making swords, that were quicker and cheaper. So swords could be produced in larger quantities, and quicker, so they started to become the weapon of choice around then for most armies, foregoing the more traditional spears and halberds as used by older armies. By the end of the Middle Ages, swords were a common weapon, and by the time of the Renaissance, they were used by almost everyone as a weapon of personal safety and protection, right up until the end of the 1800s and early 1900s, when they were all but replaced by guns as firearms became more common in manufacturing and usage.
So really, using swords as a common weapon in RPGs fits, historically speaking. And there are plenty of RPGs anyway where you can still use things like axes, though not so much halberds or polearms. That said, most RPGs take place in different fictional worlds anyway, so the development of weapons technology could easily be vastly different to the way it went on Earth, in real life. Thus explaining why non-sword weapons are excluded so much...