Because One Piece is a World that is alive, and the events going on are constantly in motion, regardless if he spends the majority of the time focusing on the Straw Hats. Most other shonens are quick to send their characters on these timeskips, and when they meet back up, it's like nothing else happened in the World while they were away. They resume as if they were picking up right where they left off.
That is not the case with One Piece. In order for Oda to set up a two year timeskip, he had one of the Great Superpowers of the World wiped out, and it's leader killed, and another great Superpower suffer heavy losses, and have to re-build. This gave the Straw Hats a decent amount of time to train and become stronger, without the World around gaining to much of an advantage over them.
Whitebeard was dubbed the King who sat in front of the Throne, but never upon it. He was the giant obstacle that blocked anyone else from making progress towards Raftel. After his death, it opened up the floodgates for all these powerful individuals to make preparations to make a dash for the throne.
The second issue we run into in regards of letting the Straw Hats become much older, is that all the powerful roadblocks that would stand in their way, are already old to begin with. If Oda did a 10+ year time skip, then most of these characters would be past their prime, and Luffy and the Straw Hats would be way too powerful. There would literally be no more room for them to grow and advance, and the second half of One Piece would be the Straw Hats journey of them coasting to the finish line, demolishing everyone in their path.
This is why I despise most timeskips in general when it comes to Shonen, because it's like nothing else happens to expand and grow the characters outside of the protagonists. Everything just stopped, while they are sent off the get stronger. One Piece is the only series to do this right, but had it been any longer than just the two years Oda gave them, then it would not have worked at all, because the rest of the World doesn't stop in One Piece, just so the protags can go off and train.