Psalms 78:39 - Joseph Benson’s Commentary on the Old and New Testaments

For he remembered they were but flesh He considered the corruption of their nature, which inclined them to evil, and was pleased to make that a reason for his sparing them. See the same argument used to a like purpose, Genesis 8:21. Or, rather, flesh here signifies the frailty and infirmity of their nature, as the next clause seems to interpret this. He considered how weak, and frail, and short-lived they were, and that they could not continue long, but would die of themselves, and moulder into dust; and that if he did not restrain his wrath, but proceeded to destroy any considerable number of them, the whole nation must soon become extinct, and the promises to Abraham and the other patriarchs fail of accomplishment. A wind that passeth away, and cometh not again That are quickly cut off, and when once they are dead never return to this life.