>so why not go for a nice central-custodian system like that run by VISA, Mastercard, or your friendly neighbourhood bank which is protected by the state, rather than for one that relies on the fact that protecting it wastes so much money that every attack will be very costly (if this was not clear, I have explained this in detail here)

This article brings up a good point about consumption, but this introductory question is dumb. The short answer is "because that's the entire point".