Raftel is unreachable because it's surrounded by an enormous area where weather, currents, winds... every possible factor is totally unpredictable: meteorological events happen quickly, and they're so powerful that even Roger wouldn't survive into one single storm. Plus, Raftel has no magnetism, and cannot be seen even from sky islands (because clouds).
But there's indeed a way to Raftel. To even try to breach into the vast area surrounding the island's probable position, you must build enormous dikes in various areas of the second part of the Grand Line: this will alter some sea currents and open a thin, crazily convoluted (but less deadly) seaway for the last island. But here's the trick: If someone manages to destroy even one of the dikes, the seaway disappear.
So, if you want to go to Raftel and come back alive, you need to prevent that someone will use this opportunity to kill a powerful crew with ease.
That's why the Four Emperors want to control vast areas, maybe you need to contol all the second half of the Grand Line to reach the One Piece.
And... What if the "Tequila Wolf Bridge" is built in something like "kairosekigranite" (like ferroconcrete, but antiatomic!

), and World Nobles are trying to reach Raftel with it? So they can finally take their time to snatch away what it needs to be taken, then they'll destroy any building and sink the island. (like the real life "Salting the earth" pratice)