I worked for Apple Support over the phone for a while and had to deal with people's photos issues. And let me tell you that the way iCloud photo syncing works is basically only to lock people in. It's just convoluted enough that the average user won't have a good idea of where exactly their photos are stored. Especially when they run out of space on their phone (very easy to do when 128gb is still the base storage space). The UI makes it seem like the only option is to buy iCloud storage and once you're paying for storage then it really makes it unclear how you could access your photos on anything other than an Apple device.

That alone is enough to lock most users in to the ecosystem. The idea of figuring out how to transfer all your photos to an Android device is too much work for most people.