> The only reason people don't like talking to computers in public is that ...
It does not seem right to speak of a single reason. There are probably multiple. So, IMHO it would be more productive to come up with a list and put some weights on the options if you want to dissect this matter.
IMHO one very strong factor / important reason (one that you ignore) is the social context. Ie the reaction of others in the same physical space, as you start talking out loud, seemingly unmotivated.
Humans are social animals, and so the reaction of others to the actions you do tend to be very important to a large fraction of the population. What is acceptable in one context simply isn't in another. Also, the exact tolerances tend to differ with the local culture (here "local" is used in the sense "geographically/physically local")
It's not just about not annoying others here. In this case it's also about a thing as imprecise as "perceived self image". Some people (I'd argue, most people) dislike having the perception that others perceive them to be mentally unstable or rude. Most people need some kind of social acceptance for the actions they do.
One significant trait of some mental instabilities (as well as some drug induced behavioral changes) is that those affected will spontanously start talking in public. You will probably know the Tourettes Syndrome, and the alchoholic rambling about because these cases often imply quite rude and offensive verbiage and/or loud volume, but these are not the only cases.
People in general are well adept at detecting such anomalous behaviour as it is part of our insticts trained through Evolution. Also the uncomfortable feelings that observing this type of behaviour leads to will lead many to react with a "confront or escape" (aka. "fight or flee") response (a stress signal), which is not beneficial to social interaction in general.
TL;DR: If you speak out in public without a very clear and socially valid reason (speaking to an object is not that) you are not only rude to others, but you also cause them stress... and you will have to face the social stigma of being perceived as insane.
(edit: grammar/typos)