Unification in first order logic

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I know unification problem in first order logic is defined as a equation $t_1=s_1, ..., t_n=s_n$. My question is that why unification in first order logic can not be defined as a unification of a formula? Why should unification in first order logic be limited to unification of first order terms? In first order logic, when unification is defined as a equation, it does not consider unification of formula which contains quantifier. In this case, why unification type of first order logic is unitary?