Would you like me to explain?

a) Confirmation bias. Not entirely, but a large part of it is just that you expect weird things from Japan, therefore when you see weird things that come from there you assume it's because it's Japan, not because it's just weird people making stuff.

b) This stuff is weird in Japan too. 95% of Japanese people have no idea that their country has this reputation, and are unaware of 99% of the things you've seen. The funny thing is that if you tried to show Japanese people the stuff that you've probably seen they would think you were weird for knowing about it. They wouldn't think of it as being a representation of their culture, just something weird.

Japanese people have these really dated/innocent ideas of how other nations view them. They think we imagine a land of samurai and sushi, and traditional things. They'll ask you questions like "can you eat rice?" or "can you eat fish?". If you mention anime they'll probably say "do you like Dragonball?" and be surprised if you read something even as well-known as One Piece or Bleach.

For me the weirdness is more subtly in mainstream stuff; that's where you find out more about a culture. But that's different from what most people think is "weird Japan" which is mostly niche stuff even in Japan.

tl;dr For the vast majority of Japanese people it isn't their culture and they have almost no awareness of it.