Because people never speak words to into their phones...
Yes I know what you are getting at, but unless you are within earshot it's not like you will know they are talking to an automated system and not a real person. Keep in mind the best use case for a system like Siri or it's Android counterpart is in the car while driving.
For me the biggest problem with Siri is I am not sure I could ever get used to using it. It will be interesting if I feel differently once I get the iPhone 5 and can put it into use while on the road. It's the generations growing up with this kind of tech that will find it most useful. To them it will simply be a case of how things work.