The fact that, evolutionarily speaking, it's easier to find lifeforms that have a strong survival instinct than lifeforms that question or reject it can be explained with two main reasons. First, assuming that evolution is performed by random adaptations that are then tested by the environment to see if they survive and their wielders reproduce, a lifeform that prioritizes a single task is more statistically probable to develop first than one that performs complex analysis of the environment and then chooses a reaction. Which is to say, dogmatic behavior developed long before critical thinking was even possible, and by the time the latter appeared, the survival instinct was already well baked into the very structure of the brain. Second, beings that prioritize survival tend to live longer than those that end up questioning the survival instinct, as the latter die more often for obvious reasons.