I think I'm pretty close to the end of the game. I've cleared areas A1 through E4 and explored all the landmarks in each of them. I've also cleared F1, Dragon's Cavern, and am currently exploring F1's pyramid. In total, I've explored 58 areas, and fully charted 54 of them. By now I'm swimming in obsidian - everyone has an obsidian weapon, everyone has an obsidian shield or off-hand item, and almost everyone has obsidian armor.

But so far the best bow I've found is a platinum short bow, which of course I duplicated so everyone with archery training can have one.

Just curious - is my luck bad, or are good bows really rare in this game?

It's been a very long time since I played MM3 for long enough to get up to the obsidian items, but I'm pretty sure I remember picking up an obsidian bow or two in the Minotaur Maze.

I think your luck is slightly bad ... but bear in mind that an obsidian bow is a pretty specific thing to ask the random item generator to come up with for you. With armour or melee weapons, pretty much anything will do; especially for your fighter types. But if you need a bow, there's only a very few items that will do.