Why Japanese not spell Onyomi kanji characters in the same way as Chinese ?

Some Japanese Onyomi spelling is based of archaic Chinese pronunciation (middle Chinese) and some dialects of eastern Chinese. These archaic Chinese pronunciation and eastern Chinese dialects have some difference with Mandarin and northern Chinese dialects pronunciations, so they are looks different.

*Example characters pronouncing same in Chinese Mandarin and Japanese Onyomi.

爱 Ai is pronouncing same in Chinese Mandarin and Japanese Onyomi Ai.

新 Xin is pronouncing same in Chinese Mandarin and Japanese Onyomi Shin.

心 Xin is pronouncing same in Chinese Mandarin and Japanese Onyomi Shin.

安 An is pronouncing same in Chinese Mandarin and Japanese Onyomi An.

Example: 安心してください An shin-shite kudasai Please don't worry.

茶 Cha is pronouncing same in Chinese Mandarin and Japanese Onyomi Cha.

三 San is pronouncing same in Chinese Mandarin and Japanese Onyomi San.

*Example characters pronouncing different in Chinese Mandarin and Japanese Onyomi but have similar pronunciation with some Chinese eastern dialects.

二 in Chinese Mandarin is pronouncing Er but in Hokkienese is pronounce Ni and Cantonese is pronounce Ji similar with Japanese Onyomi Ni and Ji.

五 in Chinese Mandarin is pronouncing Wu but in Hokkienese and many eastern Chinese dialects is pronouncing Ngo or Go similar like Japanese Onyomi Go.

世界 in Chinese Mandarin is Shijie but in Hokkienese and Chinese eastern dialects tend to pronouncing it as Sekai, similar with Japanese Onyomi Sekai.

日本 in Chinese Mandarin is Riben similar with English pronunciation Japan. In Hokkienese it's tend to pronouncing as Nippon similar with Japanese Onyomi Nippon.