Beauty and harmony is in all Creation, as it doesn’t originate of automatic evolution which would bring forth cancer-like ugly proliferation, but of God’s will. Look at the beauty of any plant or animal, magnify it’s tiniest detail up to a single cell, and you still find amazing beauty. That’s because God is good and God is love, which is not at all self-evident to the understanding of God of our ancestors - such as the sage of Zeus and the Gods, who had all human weaknesses and wickednesses.
Inharmony, evil and ugliness was brought into the world by sin, which we must not load upon our earliest ancestors, but upon us now and today, as we keep sinning anew.
The truth is beauty and beautiful, because the truth only will endure, as it’s Divine Law; it’s God, it’s Jesus Christ Himself as He said in John 14,6: I am the way, and the truth, and the life. Edith Stein said: „God is the Truth. He who seeks the truth, seeks God - disregarding if he knows this fact or not“.
St. Paul, the Prince of the Apostles said in Romans 1,19 that God in the beauty of His creation has shown His truth to all; so disbelievers are without excuse! Jesus Himself said before in John 15,22, that the earth has seen and heard His truth (“he who saw Me has seen the Father”), so any ignorance about God is inexcusable.
Though God created the world in beauty and harmony, human kind who teaches that all that is arose of mere „big bang-evolution“ altered God’s law of beauty into his own disharmony of the lie and we did so in cooperation with the father of the lie (John 8,44) which brought the world into agony.
Therefore the question "Why is beauty truth?“ bears an easy answer:
Yours
Bruno