Why are collections of all etale-maps sets?

Question

Let $\mathbb{S} = (E,p,X)$ and $\mathbb{S'} = (E',p,X)$ be etale-sheaves over a topological space $X$. We write ${\rm Hom}_{et}(\mathbb{S},\mathbb{S'})$ for the set of all etale-maps between $\mathbb{S}$ and $\mathbb{S'}$. But why should ${\rm Hom}_{et}(\mathbb{S},\mathbb{S'})$ be a set?