But it is effectively impossible on Android as well. Let's ignore for a minute the fact that practically no one can install a custom ROM.
The bigger problem is that a huge bunch of software running on the phone is fully proprietary and closed source, and there are many many different versions for different phones around - making it virtually impossible to do any meaningful reverse engineering. So sure, your main OS may be up to date, but the baseband OS and virtually all of the device drivers will be left vulnerable, and they have just as much if not more access to the data on your device.
The bigger problem is that a huge bunch of software running on the phone is fully proprietary and closed source, and there are many many different versions for different phones around - making it virtually impossible to do any meaningful reverse engineering. So sure, your main OS may be up to date, but the baseband OS and virtually all of the device drivers will be left vulnerable, and they have just as much if not more access to the data on your device.