Because the spooks don'd want that

Some facts; Any iPhone can be opened, it just takes some time and you need to physically have the device in your possession. Of course both Apple and the feds know this.

Now both parties also have nothing against the popular belief that the devices are non-hackable; Apple wants to promote their devices and FBI has nothing against people believing that their phones are secure.

The FBI's real agenda is to get Apple to install such a backdoor into their devices that opening can be done really quick, with just a moments covert access to the device or preferably if it could be done by remote connection.