In one sense perfection is an absolute, its definition uncompromising, by terms either with or without defect. But it also means “finished” or “complete” and may be qualified, and this is how the word is used in the Constitution.
The phrase more perfect in the preamble expresses in a relative way our union’s place on a continuum, as James Madison noted during the constitutional convention:
“In a word; the two extremes before us are a perfect separation & a perfect incorporation, of the 13 States. In the first case they would be independent nations subject to no law, but the law of nations. In the last, they would be mere counties of one entire republic, subject to one common law.”
Why form a more perfect union? Why not an altogether perfect union?
That extreme would have meant consolidating all governmental power in one central authority, in a consummate national polity —— a “perfect incorporation” to include, of course, a national police state. The founders rejected that setup in favor of an ideal division of powers between States and Nation resulting in a technically perfect system of American jurisprudence and the freest country in history.