You forget the fact most of them are actually incompatible with each other. Slideout bluetooth keyboards, additional batteries, dual-sim adapters, and God knows what for iPhones mostly use spacer frames - why do we have to choose, when those could easily be combined as one ultimate accessory? But there are none such for iPhones. And not every buyer is a brainless consumer aka innocent sheep awaiting to get drilled by sick-minded pervert shepherd, pardon my metaphor.