Why has nobody been able to summon Exodia before Yugi? In the first episode of Yu-Gi-Oh!, Yugi manages to assemble all five pieces of Exodia, to which Kaiba says in shock:


  Exodia! It’s not possible! Nobody has ever been able to call him!


https://youtu.be/RfqNH3FoGi0

Why was it so tough to assemble Exodia, to the point where nobody had ever been able to summon him? I mean, we see a Rare Hunter do it in the Battle City arc, without really using the new rules Kaiba created. And given the number of people playing Duel Monsters, I would think somebody would have tried and succeeded before, despite the odds.

So why has nobody been able to summon Exodia before Yugi?
Two reasons: Rarity and plot.

Exodia is supposedly the strongest monster and summoning him means you win. In normal circumstances, summoning Exodia is very hard, and decks that can are hard to pull it off: You draw a starting hand of 5 cards out of a minimum deck of 40 cards. This means you need to draw 5 specific cards in the entire deck, which is roughly 0,00000152 or a 0,000152% chance to pull it off in the starting hand.

Decks that can achieve win safely by turn 1 need to resort in some sort of tutoring and drawing out those 5 pieces. Cards that used to help were Pot of Greed and Graceful Charity, the first drawing 2 cards (a 2:1 ratio) and the second drawing 3 but forcing you to discard 2 (1:1 ratio), yet they were both way too powerful and eventually got banned, forcing Exodia to be a random winning condition for certain rare decks.

And that's only if you actually OWN those 5 pieces. The Rare hunter owns counterfeits, and the other owners of the cards opt for other ways to use them, such as Exodia Necross.

The difficulty alone is why Yugi was most likely the first person in the future world known for summoning Exodia.

The other fact is that it is a plot point. Yugi needs to become notorious in the card game community by being essentially a rising star that is strong enough of a challenge for many. This draws more people towards Yugi, like Seto Kaiba (who was considered the strongest duelist), Pegasus and pretty much everyone else that challenges him in the series.