Cyber-Dave
Explorer
If I had to guess, I think it's because large sized weapons break the game. The game says that only large creatures can wield large sized weapons. It then provides large sized weapons that basically deal double the damage dice of their medium sized variants. That breaks the game in half and makes many large sized PCs impossible to balance. The worst offender would be a large sized fighter. Any large sized weapon user, other than the rogue, with two attacks would also be an issue. Ironically, large sized rogues wouldn't be that big of a deal... +1d8 damage on a single attack or +2d6 damage over the course of two attacks isn't all that powerful compared to what a fighter, paladin, or ranger could put out with large sized weapons. The rogues, however, would still have a relative advantage over their medium variants.
It can be done, but to do it well, you pretty much have to turn the large sized PC into a class with 3 levels that then requires you to multiclass out of it (but not before all 3 levels are taken). I created a class like that once for a homebrew Dark Sun game (Half-Giant). I kept them balanced due to the fact that it was a melee class with only simple weapon proficiency. Their damage, as a result, capped at 2d8 for the greatclub. At 4th level they could multiclass and get 4d6 greatsword damage, but they couldn't get a second melee attack until 8th level. All in all, I thought it was fairly balanced and worked for my homebrew campaign. Many players, however, hate "race as class mechanics."
It can be done, but to do it well, you pretty much have to turn the large sized PC into a class with 3 levels that then requires you to multiclass out of it (but not before all 3 levels are taken). I created a class like that once for a homebrew Dark Sun game (Half-Giant). I kept them balanced due to the fact that it was a melee class with only simple weapon proficiency. Their damage, as a result, capped at 2d8 for the greatclub. At 4th level they could multiclass and get 4d6 greatsword damage, but they couldn't get a second melee attack until 8th level. All in all, I thought it was fairly balanced and worked for my homebrew campaign. Many players, however, hate "race as class mechanics."