I think that has a lot to do with where they fall within the spectrum of Anglicanism (and Huber can correct me if I am misstating this). Anglo-Catholics are the ones looking to swim the Tiber, but evangelical Anglicans simply don't consider it a reasonable possibility because of theological differences.
The Anglican church was organized by Queen Elizabeth as a compromise attempt to end years of vicious religious warfare between Protestants and Catholics. It was deliberately designed as a wide spectrum so that it could contain almost-radical Protestants on one end and almost-Catholics on the other, while excluding Those Most Likely To Make Trouble on both ends.
This worked o.k. (other than the English Civil War, but that wasn't entirely a religious conflict) until the American church threw a monkey wrench in the works. "High" or Anglo-Catholic parishes coexisted alongside "low" or essentially Protestant parishes, and other than passing a few half-joking remarks about "smells and bells" and "Virginia clericals" (a business suit with a dog-collar), we all got along.
Now, of course, people have to decide where to go, and all the theological issues come bubbling to the surface. Unfortunately, the folks who are most likely to leave ECUSA for the same reason -- abandonment of Scripture and Tradition -- are on the opposite ends of the spectrum.
The "organized resistance" is overwhelmingly Evangelical and "low church" in orientation. There's really no place there for "high church" people. Most of the nosebleed crowd have already headed over to Rome, or in a few cases to Constantinople. The muddled middle -- many mushy liberals, others who go to an Episcopal church for social reasons, others who have just stayed because it was their grandparents' church -- aren't likely to be going anywhere. They'll go down with the ship.
Perhaps one would not describe it as swimming the Tiber as much as believing that Christ calls us to bridge it, along with the Bosphorus. As far as Anglican evangelicals and Calvinist protestants, there are many for whom, as you suggest, even waiving across the Tiber would be theologically difficult. There are also many in all camps who simply have incomplete information on the other camps.