Because talking is a function of conversation between two or more people. It isn't the act of speech as a one way carrier. I've been able to dictate to my computer for years but nevertheless still prefer to type. Dictation is actually a skill one has to learn, as in the case of people such as lawyers and actuaries who don't type their own letters, it doesn't come very naturally to most people.
Because people find the concept of a person talking to themselves in public a bit weird, and talking into a completely unthinking machine is basically that. Maybe perceptions could change when low-latency conversational AI is very widespread but I think for the medium term unless there's a second human involved, people will still instinctively see it as talking to yourself, not talking to "someone".
Siri/google/alexa generally understand less without repetition, especially as complexity increases, than our fellow human. Making it doubly annoying in public
People often discuss private topics on a train, and don't care that others might be listening.
People talk loudly on the phone when in public. Some even talk on speaker phone.
But noone ever talks to Siri or Google or Alexa in public.
Why is that?