Because of a thing called the discount rate. For future or uncertain outcomes, one rationally discounts the amount they are willing to pay right now, with 100% certainty, against the future and/or uncertain outcome.
There's a possibility (IMO, a certainty) that torture isn't 100% effective, and there's some number greater than 100 of deaths that I'd need to avoid a year from now to torture 100 people today. (some of those 100 might die anyway before the terror event; we might thwart the terror event via some other means; the terror threat may itself disappear [death of those terrorists or other means])
It's just another case of "pay me $1MM today and I promise to pay you back exactly $1MM in 20 years"...
There's a possibility (IMO, a certainty) that torture isn't 100% effective, and there's some number greater than 100 of deaths that I'd need to avoid a year from now to torture 100 people today. (some of those 100 might die anyway before the terror event; we might thwart the terror event via some other means; the terror threat may itself disappear [death of those terrorists or other means])
It's just another case of "pay me $1MM today and I promise to pay you back exactly $1MM in 20 years"...