You're right that software has a negligible marginal cost, but what you're forgetting is that it has high fixed costs. It takes significant time and skill to create quality software. And, since most custom software sells one unit, it's the fixed costs that matter.
And, as your fixed costs (hardware, hosting, etc.) gets cheaper over time, but the people remain just as expensive, why do so many companies aim at "getting big", and don't really do anything about talent dilution?