Because the overwhelming majority of computer users in the world are not sophisticated, and want things to "just work" once those things are plugged in. I don't

Because the overwhelming majority of computer users in the world are not sophisticated, and want things to "just work" once those things are plugged in. I don't think it's an unreasonable expectation/desire, despite the risks. Keep in mind that autoplay is not unique to USB drives either. CD-ROM drives have had that feature forever.