OK, thanks for the reply.
I seem to recall that certain device types, like disk drives and audio devices, have a standardized USB interface. That is, no device-specific driver is required (assuming that they design to the standard, of course). I guess then what you're saying then is the iOS devices don't have that standard audio-device driver. I guess that's a not-unreasonable choice on Apple's part.
I suppose another possibility might be somebody making a (small, light, preferably in-line) adaptor from USB audio to 8th-inch pin plug - essentially a very-small, very-simple USB-interfaced A/D-D/A). I looked for such a thing on Amazon, and didn't see anything like that, but perhaps I didn't use the right keywords. Then again, such a device would presumably need a battery, which would probably contradict the "small and light" criteria! :-)