(SPOILER)
isn't it because all three of them are unsent? We know Auron is, and Jecht is Sin, and at the end we see Tidus disappear. I think that's the main reason -- they're intrinsically dreams of the fayth, to an extent. One could argue that Seymour could go there too if he wanted, and whoever else in the ranks of Yevon who are dead.
I think it's somewhat intentional that the plot is 'shoddy.' I think it adds to the story if it's a little uneven in this context. It makes the mystery all the more interesting... and plus, we wouldn't still be talking about it if it was all laid out in stone.
I don't think you can reach dream Zanarkand by airship because it's kind of like Heaven... you can't go up in a plane and just 'keep going' until you get there. But then of course you get into religious arguments about the very existence of Heaven in the first place. I think that because it's a
dream of the fayth, such an idea is implied. The play on words is illuminating in that regard.