Wildflower gardeners love them, and it is true that most of them are not difficult to grow or transplant, and if conditions are good, they thrive. However, it does help to know the facts.
Here's how they are propagated. Trilliums such as The Great White spread very slowly by underground rootstocks, and the seed produced creates new plants even more slowly. From a planted seed, it takes approximately five to nine years for a Trillium grandiflorum plant (the Great White Trillium) to bloom! So when you see a massive drift of these in spring, you know you're looking at a bunch of plants that are at least a decade old, probably much older. Trillium plants can live for up to 25 years.
And how do they propagate themselves? Well, T. grandiflorum is one of the wildflowers whose seeds are distributed by ants. Yes, ants--not birds, or bees, or the wind, but ants. This is why the species creates large, close drifts over the years. Plants are never very far apart, since ants don't travel far. So each clump of T. grandiflorum you see was planted where you see it by an ant. (They carry the seeds away when they fall from the plant, because the ants enjoy the sticky covering each seed case has when it falls to the ground.)
That brings us to the basic rarity of the Trilliums. A big factor is that each flower produces only one seed case when it fades. (Everybody knows that most flowers--a daisy, for example, produces hundreds of loose seeds from each flower.) So even if the ants find the sticky seed case, and take it underground where the several seeds inside can grow, there simply aren't huge numbers of white trillium seeds being planted each year. Other trillium species have various propagation strategies, but all take years and years.
Now you have some idea of the value of these beautiful plants. They are an important part of American botanical history, and deserve a place of honor in every American wildflower garden.