melamine has been找 in milk why did food supplier do that?

It is because of protein. Milk should contain high level of protein to provide adequate nutrition to babies.

Buyers need the milk supplied containing adequate protein.
So buyers test the milked supplied.
The test is to study the content of nitrogen of the milk.
Melamine can increase the nitrogen content in the milk so passing the test.
Melamine is cheap comparing to cow milk.
So suppliers dilute the milk and add melamine to the milk.