When you consider the safety of a phone, you must consider the worst case, that the phone falls into the hands of a sophisticated hacker. It must be safe in that situation.
Apple is in a similar situation as a very sophisticated hacker, as far as iPhones are concerned. So to be safe from sophisticated hackers who have the physical phone in their hands, an iPhone must also be safe if it is in the hands of Apple. If Apple could decrypt the contents of the phone, then there would be a great risk that hackers could also decrypt it. Therefore the phone must be designed in such a way that Apple can't decrypt it, even if they want to.