Because being of one nationality instantly means you only speak that language and nothing else. And there totally aren't people from English speaking countries who move to Japan to possibly get a job at Nintendo.
The issue isn't that we don't believe that someone has the ability to speak both high level Japanese and English, the issue is that they are writing perfectly legible English essays with common English mistakes/ no second language mistakes/ using English-exclusive names (such as pokemon names).
The "leakers" that aren't friends of people with Namco claim to be Japanese and preface their statements with "sorry if I used incorrect words" and then continue to have the issues I listed above.
I work for a Japanese company and have done so for years. I have lived in Japan. I read Sakurai's twitter for updates (yes, in Japanese). I have NEVER met anyone that was Japanese that could speak both languages perfectly without a couple glaring errors that do not really affect the meaning of what they are writing. Even if the parents of the individual were native Japanese speakers and spoke to that person since birth in the language, but they grew up in America, they STILL have some difficulties speaking perfect Japanese.
Anecdotally, I'd also like to point out that the leakers claim to be Japanese. As in they are Japanese citizens. Japanese citizenship is literally IMPOSSIBLE for non-Japanese blooded individuals to obtain. For example; let's say that an American dude marries a Japanese woman and has a kid and the kid is born in Japan, but later the family moves to America, where the child is raised. Even though the child was born in Japan and has half Japanese blood, once they become 18 years old they must choose whether to be full Japanese citizens or American citizens. They cannot be dual citizens after age 18.
If you are a foreigner, you can NEVER become a Japanese citizen, even through marriage or other means. you can gain half citizenship, but can never be a full citizen.
Okay, so now that you know how hard it is even to say "I'm Japanese" you should be able to understand why I'm so skeptical of reading a leaker who claims "I'm Japanese" and then writes a perfectly written essay. It's not to say that it's impossible for someone to be completely natively fluent in both languages, but damnit the odds are high against it. Then add to the fact that the writers will mess up common English grammar (like it's vs its) but nail all the idioms and turns of phrase- I just cannot believe the writer to be who they say they are.
If you guys want me to translate a Japanese rumor, I will do it. I did it for Brawl and I have no problem doing it. I just don't believe the "leakers" to this point who claim they are Japanese.