They have a long history of encrypting firmware. iBoot just stopped being decrypted recently with the launch of PCC, and prior to iOS 10 the kernel was encrypted too.

The operating theory is that higher management at Apple sees this as a layer of protection. However, word on the street is that members of actual security teams at Apple want it to be unencrypted for the sake of research/openness.

Someone high up is an idiot presumably