If you regularly attended church, you may feel differently. Death is mentioned probably half a dozen times in a regular service, and is the main theme of Easter season services.
Anyway, I don’t think that it is the main motivator, but it is the one that hangs on the longest, and the one that motivates squeezing god, like putty, into any cracks in our understanding. Where did the universe come from, where did we come from, all these questions can be addressed, and maybe one day even answered, with science.
What happens to our “souls” when we die is something that likely will never be answered by science, or at least not to the satisfaction of someone who wants a particular answer, anyway.
After 400 years, we still debate the merits of Pascal’s Wager, which is entirely about the possibility of an afterlife.
Tell a YEC about evolution, and they will think you are full of crap. Tell a theist that there is no god, and they will happily argue with you for hours. Tell a Christian that they are a soulless bag of complex chemical reactions that will one day simply cease to be, and they will be existentially offended.
The whole threat of punishment for going against god’s will is wrapped up in an afterlife. In this life, we see liars and cheaters and worse rewarded with wealth and luxury. In order for us to feel fair about this, we need to know that we will be the ones watching them suffering in misery and squalor one day, and if not in this life, when?
I will acknowledge that this is likely more of an American POV, with our history and culture being strongly molded by Calvinists and Puritans and other Christian sects that came to the New World in order to engage in religious persecution free from government interference. We are guided more by W. W. Norton’s sermon, “Sinners in the Hands of an Angry God” than J. H. Christ’s “Sermon on the Mount.”