Ultimately the whole thing boils down to enforcement. If there were no consequences to me walking into a shop, picking up a laptop and walking out why would I ever pay for one? The same is true for software, if everyone could pirate software with no consequences at all then there would be no disincentive everyone not to do it. You could argue that if everyone did this then everyone would lose out as the developers would go out of business and there would be no software for anyone, however that's just an example of the tragedy of the commons.
The situation we have now is one of Risk vs Reward, if you can't afford something you could try and steal it and you might get caught, as you can't afford it your only choices are to take the risk or not have it, the willingness to take the risk vary depending on perceived need. Whereas if you can afford it, but choose to steal it so you can spend the money on something else then you are taking a bigger risk because the consequences to you are more severe, after all you can't take money away from a beggar that has none.
Back to my original point, the BSA and their ilk are there to make sure the people who DO buy the product continue to do so rather than try and cut corners. They make the big noise about lost revenue because they need those figures to make sure they can convince governments to keep the consequences harsh.