Question: For those aware of the existence of rebirth, why do some still cling on to their lives in the face of real danger, as often portrayed in movies?
Answer: Here are some possible reasons —  Most movie characters are not written to believe in rebirth.  Even if one believes in rebirth, it is a strong habitual tendency to 贪生怕死 (crave for life and fear death) due to these reasons — [i] Reluctance to let go of the cherished in this life (out of attachment). [ii] Fear of the pain of death and the unknown thereafter (out of aversion). Both existential attachment and aversion arise from delusion.
Question: I agree that life is precious and we must treasure it. But as most of us are good human beings, it is highly likely that we will be reborn human beings. So, in the face of real danger, we should be able to just let go?
Answer: Are most of us good humans? Technically speaking, the fact that we are still stuck in Samsara in this Dharma-Ending Age predicted by the Buddha means the average human nowadays is increasingly losing connection with the Dharma. How strong this connection is in terms of understanding and practice determines how good we really are. Many humans these days do not observe the Five Precepts well, without which can lead to the loss of the human rebirth. (There will be a course called ‘Project Precepts’ soon, exploring how to observe the Five Precepts well, to be listed at https://thedailyenlightenment.com/category/06)
That said, even regaining a human rebirth in this world is not safe as this world is entering the Dharma-Ending Age further as we speak, with morality generally worsening. This makes it easier to lose track of observing the Five Precepts. Also, there is this recurring existential problem of three lifetimes: https://thedailyenlightenment.com/2010/08/are-you-recycling-yourself-every-three-lifetimes This is why the Buddha urged the reaching of Amituofo’s (Amitabha Buddha) Pure Land, so as to overcome these problems once and for all, as there can only be non-retrogressible progress to Buddhahood there: https://thedailyenlightenment.com/2014/03/destination-pureland-how-to-die-well-be-reborn-better (Next run of this course will be in 2015.)