If I'm taling to my phone, I'm not looking at the screen for ads. So how does Google bring ads to voice-recog? Is the voice reply interrupted by voice ads? I wouldn't use it.
Doesn't have to be ads. Suppose you run a restaurant. When someone asks their phone about restaurants nearby, how much would you pay to have your restaurant preferentially mentioned before other restaurants? How much would you pay to get access to a list of questions people ask about restaurant-related terms near you, and how the person reacted to the results? Did they ask further questions about your restaurant? Did they view your page? Did their phone's GPS indicate that they visited your establishment?
I'm assuming you are referring to Android. It currently has many features that don't drive ads. How much Android makes for Google and how it does that is a good question though.
Not only that, but Siri acts as a layer of indirection between you and the search engine. She can decide which search engine to use based on the query. That opens the door for subject-specific search engines to steal chunks of traffic from Google without forcing users use a bunch of different sites and interfaces.
Just because you're not seeing ads for a particular use case, doesn't mean that the data is not valuable for some use case in the future where you will be seeing ads.
It's Google's interest to make the best, easiest way for you to get your personal data in and out of their system.