Because metals are often very reactive, as are acid.
Metals will react rapidly with most acids. If the acid is concentrated the reaction can be violent. To react one with the other simply drop acid onto the metal surface with an eye dropper or place the metal object into the acid (with care if the acid is strong).
acid
Citrus foods are not stored in metal containers because the the acid in the citric foods react to metal.
when metal react with an acid it forms a salt and hydrogen gas
GOLD
A metal that does not react to acid, oxygen or water does not exist.
When carboxylic acid react with metal gives Salt and hydrogen
Copper metal does not have enough reactivity to react with sulfuric acid.
Some do, some don't.
elementas
In a typical case an acid and a metal will react to form hydrogen gas and a salt. The salt formed depends on what acid and what metal is used.
Metal carbonate Metal oxide Metal hydroxide