If you are a local you are probably very used to your country domain, it is probably on par with .com.
Now the Accept-Language is quite subtle and guess how that is set up in an internet café? Can you even change it? Will the next customer be happy about that? I'd be mighty impressed if more than 1% of the people that surfs the web has a clue about Accept-Language.
Also, if one asks for a local country domain that really should be the localized version, sometimes I actually want to use google for a language different other than my own.
If I for instance wanted to search in English on something that has a similar wording as Swedish I'd like to go to google.com in order to quickly get rid of the Swedish-bias (since that doesn't work I sometimes use google.co.uk instead).
Many people are however, and mass media, which includes Google, however they may try to deny it, lives by the numbers, which means adapting to the lowest common denominator. Just think, as the Internet spreads, Google will evolve (devolve) closer and closer to broadcast TV!
Also, if in Japan, I would be surprised to find google.com not to be in Japanese, and I would expect to have to perhaps make some kind of preparations for the fact that the majority of the world does not speak my language and that my electronic devices and services are not universal metatrons run by omnipresent deitys either.