Question

Why can not each metal react to its own salt (Eg. Cu and CuSO4) ?

Answer 1

each metal cannot react its Own salt because
the the reactive strength of the metal will be same in both example cu and cuso4.
In both the metal cu (copper) has the same reactive strenght. if in case there was fe (iron, more reactive than cu) to be reacted with cuso4 than displacement reaction would have taken place.

OR

Reaction of metal with salt is a displacement reaction. It goes like :

Metal A +salt solution of B=>metal B+salt solution of A .

In this A is more reactive than B .

Since , a metal can not displace it self so it can not react to it's own salt. For e.g. copper can not displace it self in CuSo4.