Question

why cannot each metal react to its own salt for example copper and copper sulphate

Answer 1

because it's own salt contains the same elements.so it doesnt react with it's own salt.

Answer 2

reaction of metal with salt is a displacement reaction for example metal A + salt solution =metal b + salt solution. In this Amore reactive than B since, it cannot displace itself so it cannot react for its own salt like copper cannot displace copper sulphate