Why are some words written in kanji and somr arent? For example, Ive seen the sentence, "わたしは元気です" while it should be 私は元気です", even though it has used the kanji 私 before. Why is this? It makes no sense to just have only some words in a sentence as kanji. Also, "はれ" can mean different things, so kanji would be useful in this.
Actually Japanese actually was scripted after the influence of Chinese characters , that is know as Kanji now. But, the people, especially the women(who were kept out of formal education's boundaries ) found a script based on sound that is hiragana. And for foreign words, like computer, email etc. which don't exist in Japanese, katakana is used (which has same sounds as hiragana).
For learning purpose , duo is using hiragana for both words written in Kanji or katakana. Like cat, has Kanji symbol which sounds like neku. Though the prevalent representation is in kanji (where the symbol is same as mao(cat) in mandarin)
So in Japanese you may find three different scripts in the same sentence.
I think initially the structure was to integrate vocabulary before inundating us with the slog of learning all the Kanji, but ultimately, how they've designed it to not take into account the kanji we've learned and swap out the previous hiragana as we go seems silly. I agree.
As you have seen Japanese writing has no punctual system. No commas or semicons, just pure furigana. Kanji are used so as to get different words divided, a sentence will look smaller, better to read etc. And Japanese people love Kanji. They have an attachment to it, there's no denying it.