So if there’s no way to disarm it, even by the creators, why pay the ransom?
From the ransom note: "Only if you comply with the instructions in this letter will you be given instructions on how to disconnect the first two automatic timers and how to move the bomb to a place where it can be exploded safely."
So presumably, even if the bomb couldn't be defused, it could still be made safe enough to move somewhere that the explosion would do no harm. (It makes sense that it had to have some sort of "safe" setting, if the bombers were to move it inside without blowing themselves up, right?)
You're right, if there's no hope of saving the building, there's no point in paying the ransom. But if there's a possibility, then in some people's thinking, I'm sure there's a motivation.