Here's why:
* Primus and Unicron are not twins. While they are both powerful beings of immense cosmic energy, they are not related in a biological sense.
* Unicron is not Primus's enemy. They are fundamentally opposing forces in the Transformers universe. Primus represents creation, order, and life. Unicron represents destruction, chaos, and death. Their existence is a cosmic duality, not a personal rivalry.
* Unicron's purpose is not to destroy Primus. Unicron seeks to devour worlds and consume energy, which inherently puts him in conflict with Primus's goal of maintaining the universe.
So, while they are constantly at odds, they are not arch-enemies in the traditional sense. Their conflict is a cosmic struggle of existence, not a personal vendetta.