Terabithia exists inside Lark Creek – it's brought to life within the bounds of the boring, the stifling, and the ordinary. What's amazing is how living and being inside Terabithia allows characters like Jess and Leslie to take its magical properties with them into that other, seemingly boring, straightforward world. This happens in moments like when Jess finds the same peace in the National Gallery as he does in the sacred grove, or when the two decide to help, not harm, Janice Avery. They learn that, even in the darkest moments, kindness, love, and grace can be found anywhere.
Present-day children are so consumed by elaborate toys and technology that they don't have the capability to create a magical world like Terabithia for themselves. (If you disagree, how about arguing for the other side?)
Without the dullness and lack of excitement or energy in Lark Creek, the invention of Terabithia wouldn't have been possible.