If God truly is just, then why does he not treat all people equally?

The modern idea of justice as “fairness” or “equitable treatment” should not be imposed upon the understanding of God’s justice. As noted above, the biblical concept of justice as used in Scripture (the צֶדֶק and δίκαιος word groups, respectively) is interchangeable with the concept of righteousness. To be just is to act in accordance with a standard of righteousness. God is always just. God is not always equitable.