To begin with, everything can’t be written in kanji. Kanji are the Chinese characters the Japanese use that only have meaning, but no pronunciation. I don’t speak Chinese and don’t understand most of what the Chinese write, but when I look at things written in Chinese, they have a tendency to supplement things with the English alphabet. I don’t know if they have to or just have a tendency to borrow even though they don’t need to. I have a friend who is a Chinese teacher from China who told me that Chinese didn’t have grammar until the Chinese encountered English. They didn’t have a word order, and just wrote everything just however they pleased, but now, they use the English word order. English influence changed the Chinese language, and they also add the English alphabet at times. However, I don’t know if they have to or not.
As far as Japan, they started with kanji when it was shared with them from the Bekjae people from Korea. In old Korea, the grandmothers taught the children within the confines of the courtyard the houses surrounded. The girls studied, but they weren’t allowed out of the house because of violence in the streets unless they went in one of those sedan chairs and were hidden behind curtains. Or, in old Seoul, they used to sound a bell, and for an hour, all the men had to go in the house, and only women were allowed in the streets so they could go shopping. The Korean boys were pushed in their knowledge of the Chinese characters because only the boys who knew lots of Chinese characters could get good jobs or work in the government, and it was the same in Japan. The boys were pushed to learn the kanji, but not the girls.
The ladies of the court in Japan wanted to write poetry, but they didn’t know kanji. They couldn’t write. Hiragana was invented so these women could write and use it to write poetry.
The Koreans invented hangul, their alphabet, for a whole different reason than the Japanese invented hiragana. It bothered King Sejeong that everyone in his country couldn’t read. He wanted everyone reading, so he made up an alphabet, the hangul, that they could all learn. He made his country literate by simplifying their lives because only the boys of the upper class families were able to read until he invented hangul.
The Japanese have a perfectionist nature. This showed up in their writing. Instead of wanting to invent an alphabet to help the people, to make their lives easier, they pushed all their people to learn kanji. They measure how much education you have in Japan by how many kanji you know. They are not a down to earth people like the Koreans. In Korea now a days, only the scholars use the Korean form of the Kanji, the Hanmoon. However, in Japan, they think everyone must know the kanji. However, they are conscious that not everyone can know every kanji they use. I talked to a Japanese who told me that he moved from one part of Japan to another and he had to learn a whole new set of kanji when he moved because he couldn’t read the signs in the new place. The Japanese don’t mind drawing the lines between people who know and people who don’t, and they have it in their heads that everyone should know, so push them! The Japanese are strict.
When I taught in Japan, I learned that the students in Japan have to take tests every so often to measure their level of kanji. The Japanese, however, don’t use as many kanji as the Chinese do. The Chinese really don’t have another system of writing, but the Japanese do, and if theJapanese don’t normally use the kanji for a particular word, they put in hiragana. Especially, the verb endings in Japanese, they use hiragana because the kanji are not originally Japanese, and they need a way to conjugate their verbs, so they use hiragana. Whether the kanji actually came from China or Korea, it doesn’t matter because the Kanji aren’t really Japanese, and the Japanese have simply adapted them to use them. To complete several words, they need the hiragana.
In fact, with several books, they put the hiragana beside the kanji so the reader will know how the kanji are pronounced. Someone like me whose kanji level is really low couldn’t read anything in Japanese if they didn’t put the hiragana next to the kanji. There are Japanese who also have a low level of kanji for different reasons. I read a post by a Japanese girl who lives outside of Japan, and she studies kanji because she doesn’t want to be embarrassed when she writes a letter to her mother by her low level of kanji. Only Japanese children have a low level of kanji. I can read children’s books in Japan pretty easy, but they also have a tendency to use words only used by children.
I am not sure why they invented katakana, the alphabet they write foreign words in, except that there are times in their history that they could be very ethnocentric. Or, perhaps, they were so confused by the foreign words that they were trying to figure out how to write them and let people know that they aren’t just a strange Japanese word, but a word borrowed from another country. When I was in Japan, the older people in Japan were trying to eradicate as many English words from Japanese as they could because they thought the young people were adding too many English words. They want Japanese to remain Japanese. They don’t want it to be a mixture of English and Japanese, or as my friends in Japan say, they don’t want “Janglish.”
You ask why the Japanese need hiragana and katakana, but I ask, “Why don’t they just get rid of the kanji and free their population up like the Koreans did with Hanmoon?” The Koreans use only hangul like we use only the Roman alphabet in English. In my opinion, they Japanese should use only hiragana and forget the other because they just complicate life. However, they argue that too many of their words have the same pronunciation, so they need the kanji to know the meaning. However, we have words like that in English too, but we know from context which word they are or we change one of the letters like the difference between “dear” and “deer.” They think they need the kanji to know the difference between two words that have the same pronunciation, but by just changing one letter in English, we changed the meaning. In Spanish, they have the word “manana’ which could mean “morning” or “tomorrow.” The Spanish tell the meaning from context. I am not ready for the Japanese just to use kanji, but to get rid of kanji and katakana and just use hiragana, and thereby being more practical and simplifying Japanese, that would be very intellgent.