It's just a difference in aims and circumstances. Japan has multiple train companies operating different networks, so a separate network was the only thing they could build. France accepted the compromise that high-speed TGV stations are far outside of the city areas, like airports, which makes nominal travel times fast, but the first and last miles will always be slow local trains to the city centre. Germany wanted ICE passengers to arrive at the central train station of a city, so using the existing train network at least partially is the only possibility. China has larger distances and no preexisting dense network, so building separate and dedicated is easier. Also, they maybe don't fuss about tearing down a few buildings and disowning a few home-owners who are in the way of their new dedicated high-speed route and station.