When Japan began interacting with China, Japan did not have a writing system. So Japan had to adopt Chinese characters and shoehorn it into their own language, in order to be able to trade with China and stuff. They later invented their own writing systems. But kanji kind of just continued to hang around. I imagine that part of it is just because kanji was already so ingrained that it became difficult to get rid of. It’s kind of like saying why don’t we fix English to get rid of all the weird letter combinations and make it more phonetic? It’s kind of a big undertaking.

Kanji is also very economical from a perspective of how compact it is and how quickly it can be read. You can fit a ton of information in a very small space, and you can understand the words at a glance.

There is a funny thing among Japanese learners. Ask a beginner their thoughts on kanji, and they will often complain about how difficult it is to learn, and how it doesn’t make sense to keep kanji around. Then take someone who has spent several years studying and become fairly proficient, and ask them to read a passage that is entirely in hiragana or katakana. That person will likely complain about how difficult it is to read, because they have to look at each individual character and sound out the words.