You bring up a good point, the difference is that consumers consider a computer, tablet, smartphone as main products not accessories like smart watches, wireless earphones/buds, etc. Accessory manufacturers were depending on people who use their product such as headphones will be in compliment with a smartphone, tablet, computer, etc that does (did) come with a power adapter or some form of aux port on the device to charge it. iPhone does not have a method to charge other devices but iPad Pro/Air with USB-C does as well as a computer.
The question remains does Apple now consider an iPhone as an accessory and expects consumers to charge it via iPad/Mac/Qi/MagSafe/power adapter.