Although the monsters were not defeated instantly due to their sheer numbers, their eventual demise was just a matter of time given the circumstances back then.
After being released from Zhou Tian's seal, the aliens immediately began a series of battles with the monsters. At the onset, the monsters didn't suffer significantly due to their sheer strength. However, with the abilities of the aliens, as the battle wages on, the monsters inevitably found themselves at a disadvantage.
One by one, the aliens used the monsters as hosts to grow new members of their race. By the time Zhou Tian entered the fray, the aliens' strength hadn't significantly increased, but they were certainly more powerful than when they were released from the seal.
Unlike the aliens, the monsters' strength was decreasing bit by bit. With no effective reinforcement, even though they initially put up a good fight against the aliens, the monsters unavoidably found themselves unable to resist the aliens' attacks.