The reason A to A is rare is because typically USB "A" sockets are reserved for use on USB host devices and NOT on peripherals (both devices in the OP are host devices, ie devices that can manage other USB devices).
Some hosts however can be both a host AND a peripheral, such as the one in the OP (where it can be set as an external disk), but rather than add a dedicated peripheral (USB type "B" socket), the type "A" can work as both.
So really, when this was said;
The actual response should have stated that any device designed to act as a peripheral should have a type B plug on it, rather than PCs should change.
But why are they different? I understand that they are, but I'm unsure as to why. Is it not possible to make a multi-purpose, small connector that works irrelevant of the device? Am I wanting to have my cake and eat it here?