Because it needs to be "hard to get", but no one said it needed to be fair.
Honestly, I have kind of been fine with most of the rare dungeon items, they have for the most part just been rare items with "ok" stats and special in looks, but the Hunter's Stake, that's just a punch in the gut for anyone who farmed heavily for the various weapons in Ashfall, especially the Staff of the Abyss. You just know there's a couple people out there who get it on first couple tries while the majority of people spend countless hours in vain.
At this point I have given up on it, I got all David's stuff, I got both the Beastial Wolf Claw and the Silver Crossbow numerous times and I have max Shadowslayer rank AND! have run the dungeon to try an get the stake for quite a while after getting max Shadowslayer rank.
At least once Ashfall part 3 and/or higher levels come out, there will be more options to farm (yes, farm, not some cheap low drop from a random dungeon) for weapons which will have better stats than the Hunter's Stake.
This type of RNG should never be tied to an item which is arguably, without question, the strongest of it's kind. Some people might disagree, but I think a majority of people think exactly the same.