Because sunk costs are sunk, and fixed costs are fixed. In a commoditized market, the cost of the product is most directly impacted by the marginal cost to produce one additional copy. It doesn't matter if you had $100M in your production budget, or just $1k, because all it takes to make one more copy is a few squirts of electrons and a pinch of network bandwidth.

For digital goods, marginal cost of production is so near to zero that the only thing you can really sell profitably is an artificial legal right to distribute the copies.