Unions don't confer benefits to people not in their union and actually increase costs to them by putting up barriers to entry. Nations can do the exact same thing with immigration policies with respect to citizens and non-citizens.
A union in this case would benefit the majority, but at the cost of people not in the union.
Tighter/more strict immigration would benefit the people in the country in terms of reducing downward wage pressure, but it would remove opportunity for people abroad/non-citizens.