Re: Honest Question
Someone on reddit explained it very well: part of the key is wired to the hardware of the phone.
You can't clone that part. The data has to be decrypted on that specific phone.
It's even more (much much more) complicated on later phones.
Apple has thousands of people working on this thing (the iPhone) and employs some of the smartest minds on this planet to think for every possible attack vector.
Later phones probably even shield against side-channel attacks (measuring minuscule discrepancies in power-usage etc) to help guess the key that is stored outside the control of iOS on the "secure enclave".
Today, it might be an iPhone. But what if (in a future that may be not so far away) it was possible to directly read data from the brain?
You'd use it every day at work, probably, and in your spare time. You'd think of a flower and your brain would send that image via your phone to someone else's phone and from there it would directly reach the brain. The device to enable this would be built in such a way that it wouldn't work without you giving explicit consent to this "transfer".
But what about when you committed a crime? Would law-enforcement be allowed to "tap your brain", against your will to recover details of the crime that you yourself didn't want to tell?
What if you were in a coma? Would it be OK to tap your brain? After all, when they asked you, you didn't really object ;-)
The above mentioned device would have to modified to allow overriding the protection-mechanism.
So, quite rightfully, Tim Cook takes a stand and says "it has to stop somewhere".
Because it has to.
Maybe Tim Cook thinks, this is his "Rosa Parks" moment. It was a small thing at the time. But somebody had to do it and as remarked by someone above, both sides may think this is the case of all cases to drag into the limelight.