Because as you say it is risky. The theory is sound, but it is a lot safer to do so in cooperation with the people who actually built the thing -- especially because this may be breaking new ground. (I don't work for the FBI, how do I know whether they've done it before?
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Slipping up can easily destroy the data they are looking for, beyond all hope of recovery. They're hoping to take the easy way out.
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They're litigating because they want to establish a precedent that tech companies need to help the FBI when it comes to national security matters. We've already said that.