Various estimates indicate that only a bit over half of the fiber already in the ground is currently in use. The large amount of "dark fiber" is a result of the high cost of burying it versus the cost of the fiber itself. Typically the fiber materials only amount to about 10% of the total run cost, with labor, easements, and equipment making up the rest. Because of the low materials costs, it makes sense to lay multiple fiber runs at the same time to cover both future expansion and in place spares needs. There are a lot of other costs involved in Internet capacity expansion of course...