Quotetheoretically, sponsored content could be added to voice search results with relative ease, the issue is that the real estate for voice is much smaller than it is for screens, which can feature multiple ads without impeding on the user experience. Moreover, users turn to Amazon with a specific use case in mind (purchasing a product), but Google's much larger sandbox landscape means that ads must be relevant and well targeted to users; this will be integral to Google maintaining its dominance in search.
http://www.businessinsider.com/amazon-is-flirting-with-paid-search-on-alexa-2017-1