Do you ever ask what their favorite thing is? Or what they do on their free time? 9/10 times, they will respond with "Japanese people like to..." Or "I am Japanese so Sushi is m favorite food."

Basically, what you are experiencing is conformity. You add pride on top of that because Japan is an ultra nationalistic country, and you got total confusion. Every time I get these questions asked, it just shows how much that person knows about the world. They aren't trying to pick on you. They believe that Japanese are all the same and in the same respect, Americans or whatever​ country you come from are all the same.

I have a few mistakes in my grammar, yea. But even though I am able to demonstrate my language ability in uncommon topics and have meaningful conversations, I still get asked if all Americans do this or that. Why? Because they are just programmed that way.

Most, if not, all Westerners believe in individualism no matter there race, which is why you are taking offense to what they are asking you. You are taking it as an offense because they are not recognizing you as the special snowflake that you are.

Lastly, one thing that you have to understand is that Japanese people study languages really hard. And even though they are able to master the language to a certain ability at school, unfortunately, they don't pick up on one particular culture since there many native English speaking countries that have influence in Japan. Educate them and don't be a prick about it.