For those wondering why the gamecock will fight, this Phillippino article tells us why. According to them, in a country where this brutal contest between rooste ... rs is legal, the humans want more than a "normal rooster". They will use methods such as, cross breeding, and training to produce "distinctive physical and behavioral traits".
It's not about the rooster, it's about the training. It's not something defensible due to rooster nature. It's just "training" for gambling pleasure. There is no rooster born with "warrior instinct". They are bred to be unsustainable in a natural flock setting, for the purpose of being disposable in a cockfighting pit.