Question

why can not each metal react to it own salt (eg.cu and cuso4)

Answer 1

For a reaction to take place there must be a difference in reactivity level.

That is one must be more reactive than the other.

If they have the same reactivity then there is no reaction.

That is why a metal cannot react with its own salt.

Like Cu cannot react with CuSO4.

Answer 2

A reaction only happens when the two reactants have reactivity level difference between them.

Same applies for a metal and it's metallic salt.If the metal and it's metallic salt have reactivity level difference between them,they will surely perform a chemical reaction.

But metal like Cu and it's metallic salt CuSO4 doesn't have much reactivity level difference between them, that's why they don't react with each other.