Because, as far as the teaching requirements of the first semester of senior one are concerned, Chinese requires literacy of 300 and writing 100. Mathematics requires addition and subtraction within 20 minutes. Chinese also needs to master all Chinese Pinyin. Generally speaking, the requirements of Chinese are higher than those of mathematics.
Moreover, reading more in advance has many advantages, so you can start reading earlier; Learning to examine questions is especially beneficial to other subjects, such as mathematics.
Of course, it would be best if we could work together. The addition and subtraction of mathematics need constant practical training from knowledge to proficiency.
Both are important. After a child enters a large class, it is easy to learn addition and subtraction from literacy.
Literacy is more important.