To do that, you'd need to disassemble the phone to insert your implant. That might be hard to do in the field (ie. not in a repair shop/lab setting with plenty of tools lying around). Not to mention the difficulties of designing and manufacturing an implant. How are you going to get it to fit? I don't think there's a lot of empty space inside a phone. How many variants would you need to design and carry around? I'd imagine that the iPhone SE would need a different implant than the iPhone XS, for example.
A bootrom attack allows you to replace all of that with plugging in your victim's device into your "hackbox" for 10 seconds. Vastly simpler to execute for your typical goon/henchmen and way less likely to get detected.