Why is a short message seventy words? Who stipulated this?

This is stipulated in the communication protocol and has nothing to do with China Mobile and China Unicom. Whoever does it will do it according to the current communication protocol. It must be a short message with only 140 bytes, which means 70 Chinese characters.

Nowadays, many mobile phones can support long short messages, but they are also reassembled after receiving them. If the other party does not support long messages, it will receive many disassembled messages.