Why does magnesium react readily with acids?

Magnesium reacts readily with most acids because it is a powerful reducing agent.12

In chemical terms, magnesium atoms easily lose their two outermost s electrons to form stable, doubly charged $M{g}^{2+}$ ions. When magnesium comes into contact with most acids, this high reactivity allows the metal to displace hydrogen from the acid, resulting in the liberation of hydrogen gas.21

While magnesium is generally stable in air and water at normal temperatures due to the formation of a thin, protective oxide skin, this layer does not provide the same level of resistance against most acids as it does against alkalies. Notable exceptions to this reactivity include chromic and hydrofluoric acids, which do not attack the metal as readily.1