I think Isono-san's question is very valid.
I would like to answer in my own way.
In the first place, sutras are Buddha's sermons, and the disciples who heard them gave this kind of sermon after the Buddha died, so many disciples gathered and compiled them.
Therefore, it is based on the Indian language of the time. (Sanskrit or Pali)
It went to China, and it was translated into Chinese so that it could be understood in China, mainly borrowing the sounds of the kanji.
When they came to Japan, it seems that monks learned from people who can decipher sutras in the form of how to read them as a Chinese calligraphy.
In that sense, the Chinese literature reading can probably be called a Japanese translation for the time.
The reason you read it as it is is probably because things adapted to kanbun kun reading are difficult to use as recitation, even though they are used for study.
Reading sutras is also called a statement (shomyo), and it is also called a kind of Japanese music.
The fact that Japanese translation is difficult to fit into the rhythm of the statement is probably one reason.
As Isono-san points out, Buddhism is for living people, and there is certainly an aspect where it is difficult to convey if you don't understand the meaning.
My temple is the Jodo Shinshu sect, and the Shinshu religious groups are jointly trying to put it in the form of a Japanese translation (translation) called “Japanese translation of Masanobu.”
However, the reason it doesn't spread is that it seems that there are voices saying that they don't feel grateful if it's not in Chinese, it's hard to recite, and they aren't used to it.
There is inevitably a sense of incongruity in paraphrasing that it is different from previous recitation, and since definitive paraphrase work has not been decided, this does not mean that there are no questions about whether the meaning of the sutras can be conveyed correctly.
It is also called “Sutra religion is a mirror,” and we search for our own truth in sutras.
In that sense, even if the recitation is in Chinese, I think it may be important in a slightly different sense for yourself to listen to the Dharma or read a book and face the sutras yourself.