Also this is one of the reasons that we can not be allowed to access One Piece. We can out bid the One Piece which completely ruins the story.
What "One Piece" actually is has been left ambiguous, but it has been implied to be more than just gold and jewels. Roger was likely a pirate who didn't value money as much as he did things he was emotionally attached to. Most major pirates in the setting don't seem to think it's just treastes, either, since most of the stronger pirates can get all the money they might want through protection-rackets, tributes, slaving, looting, and, of course, piracy. Yet, many of them, despite being apathetic towards earning money in several cases, still pursue One Piece.
For some, possibly most, what the treasure is doesn't matter. What matters is the difficulty in finding it, and the prestige, power, and fame that comes with achieving such a dangerous and difficult task. Reaching Raftel is a feat only one ship, one crew, in the recorded history of their world has ever achieved. Matching arguably the greatest feat the King of Pirates ever managed is the real goal, for many of those going after One Piece. The treasure is just verification.
The fact that one person who was in on the secrets of it said it was important enough to cause a world-wide conflict and shake the entire world is also a driving force, for some. That sort of power can be enticing for powerful pirates, and the same is true of sowing that much chaos.
We can also fly at incredible speeds - even with the world of One Piece being as huge as it is, we'd get to the end of Grand Line pretty quickly.
We aren't accessing any new planes for a very very long time though, if ever.
Raftel's location is apparently harder to find that just sailing to the end of the Grand Line. And it's hidden well enough that, so far as we know, no random ship has found it while traveling. Given that the entire Grand Line would probably be overflowing with mana, and magical phenomena (it'd explain the individual island's microclimes, the bizarre weather, the strange flora and fauna, and the effects on navigation), finding it might actually be a bit challenging, even for us. Not super challenging, but slightly more difficult than most things.
Gift of Tongues - casually read ancient languages, be it that of MGLN's Al-Hazard, Thedas' Elvish or One Piece's Poneglyphs.
Finding them could actually be a bit challenging for us. Drones might not be too happy to operating there, or may have issues with long-distance communication, due to the nutty weather and the same effect that makes compasses worthless there. We might actually have to do some legwork ourselves.
Honestly, I kinda want to see someone cross One Piece and MtG now. While Devil Fruits aren't super compatible, the entire insanity of the environment of the Grand Line could be so much more easily explained that it's almost beautiful. You could even align the islands with different colors of mana, and make the storms and crazy weather be caused by the mana from different islands colliding. Ork taking it even further, you could make a whole mana-based weather system, with fronts of colored mana instead of hot or cold fronts.