The battle of Heaven Punishment, a grand war between two realms, caused widespread devastation. Even the Martial God was but cannon fodder at the lowest tier.
Because this war would often involve combatants above the Heavenly Divine Realm, even Holy Realm Gods might join the fight. When beings of such caliber made a move, they could easily shatter the void and affect vast territories spanning tens of thousands of miles. A Martial God, caught in a mere ripple of their aftershocks, would find only death, with no chance of escape.
Though Ye Ao had grown stronger than before, he had only just stepped into the Martial God Realm. He couldn't truly influence the entirety of the Heaven Punishment battle. Even if his power could rival the Heavenly God Ye Chen, he could still not participate in the war. It was somewhat selfish, but because he was Ye Chen's father, his presence was essential.
Sometimes, the reasons are just that simple.