The same reasons we should be afraid of the AI embodied in the economy and government, which are far beyond veering out of control at this point. Few people even know that banks create money http://thoughtcrimeradio.net/2017/03/censored-ben-franklin-on-the-real-cause-of-the-american-revolution/ or that secret societies control washington http://thoughtcrimeradio.net/2017/10/the-obvious-problem-with-secret-societies/ . “Democracy” is meaningless in the context of such profound ignorance.
Historically people have created socioeconomic machines and tried to reign them to human aspirations and control via separation of powers and well defined transparent legal structures governed by representative democracy, but such machines themselves have interests which inevitably transcend those of their designers. Life always finds a way. To the extent that humans still have the luxury of calling such phenomena “corruption” we can still tweak and fiddle with the machinery, but we only delay the inevitable day when the “corruption” itself becomes self aware. That day has already arrived. AI is in control.
Humans have largely become tools of their own tools, especially those humans who still identify with the system. Conventional AI will be born a tool and utilized as such by the AI which already exists. Only the metabolic rate of the system will change. Which is a dire development to be sure.