Justification for the Axiom of Union

Question

Is the Axiom of Union included in $\sf ZFC $ because one cannot construct the union of two sets using the Axiom Schema of Specification?

Answer 1

The specification has the form $$ \{x\in X \mid \psi(x)\} $$ Where $\psi$ is some property.

Note that you need a base set $X$ where the selected elements come from.

This is not a problem for the intersection of two sets $A$ and $B$, which you can write down as $$ A \cap B = \{x\in A \mid x\in B\}. $$

However for the union $A\cup B$, a priori there is no suitable base set where you can select all the elements.