Because a rooted Android device with a Google-free AOSP distribution is (mostly - radio firmware being the exception) 'my phone' while an 'iPhone' is not. On the Android I get to decide what software gets installed, what services to run and which to shun, which programs to allow network access with fine-grained control, both in- and outgoing traffic, which versions of the software to run, etcetera. With the iPhone the choice is rather limited and the controls are mostly in the hands of Apple. While some seem to consider Apple to be a benevolent dictator I'd rather avoid dictators altogether, apart from my doubts about their benevolence.