And? This game isn't geared around loot collecting or grinding hoping to get that weapon to beat x boss or y mob. The RPG aspect isn't about grinding, it is about environment and story. Something sorely lacking in today's games.
And, the Zwiehander is certainly not a weapon one just picks up and thinks "now, this is something I could go the whole game with!" on their first playthrough. In fact, I highly doubt you did that because it is a rather unwieldy weapon. Not only does it require high stats, it is slow and has a lot of recovery between swings. Something that will get a noob destroyed by the most basic of enemies. It requires a certain style of play that people might not enjoy. Some prefer a more standard sword and board, while others like katanas or spears. Only experienced players end up with ultra greatswords and polearms, as they rely a lot on you dodging and precise timing to not get wrecked.
Not to mention the fact that, Zweihander +15 is a superior weapon to vanilla Zweihander and cannot be obtained until endgame, so that blows the whole "best weapon can be found at the beginning" argument clear out of the water.
Dark Souls is more about knowing how your weapons and moves work, knowing how your enemies' moves work, and being able to read and react. It's similar to Punch-Out in that regard. It is NOT a game about progression grinding your way through content. There's plenty of Youtube videos from good players who can clear the entire game with their bare fists or a dagger or some other crap weapon because it's all about reading and reacting to what moves are being telegraphed by the boss.
Honestly, if you liked Punch-Out, I think you'd like Dark Souls. Both are kind of pointless endeavors because it's all about learning to master the game.
If you're the type of person who is like, "What kind of idiot spends 100+ hours of their life to master Dark Souls/Punch Out?" You won't like Dark Souls.
If you're the type of person who spent 100+ hours to master Punch Out and exclaimed "I DID IT!" (PRAISE THE SUN), you might like Dark Souls.
Edit: And for those of you who played and enjoyed Dark Souls, I recommend watching Edge of Tomorrow. It is pretty much "Dark Souls: The Movie."