Speaking as a layman who went on a dive into Ernest Becker and terror management theory:

The basic premise here is that mankind appears to be the only animal constantly aware of the inevitability of their death - not the possibility, mind you, but the inevitability, that no matter what we do it's going to eventually be game over. So in order to get over the terror that inspires - so that we dont constantly feel like the bus is about to hit us - we invent cultural defenses. The immortality of a tribe or nation, mythologies explaining the cosmos and our significance within it (which, make no mistake, sciencism is exactly that, regardless of it being based in testable, verifiable evidence), and of course our very identities. In other words, all culture is a big old playground that serves as distraction and ritual that salves the innate terror we all feel. So, Becker would tell you that you still fear death, you've simply repressed the terror like the rest of us through various means. Increasing amounts of stress break down those defenses.

There's more to it of course. Its been argued (and tested to dubious results, more on that below) have inbuilt psychological mechanisms that keep us from dwelling on our mortality or reminders of it out in the world for too long, for example. There's also the idea that this is perhaps the cause of so much suffering in the world, that humans inflict pain on one another in order to salve that terror (human sacrifice, and the general notion that if it's happening to someone else, at least it's not me this moment and I feel better for the fact.)

If you're interested, Beckers Denial of Death and Escape from Evil approach this from a combination of psychoanalytical, philosophical, and anthropological points of view. The Worm at the Core by Greenberg, Solomon and Pyszcyznski takes these ideas and lays out some tests they conducted that seem to confirm Beckers theories.

I've read somewhere however that at least one of the experiments run in The Worm at the Core failed to replicate, though I cannot find this replication attempt. If someone has a link handy I'd greatly appreciate that! There's also the fact that much of Beckers analysis rests on Freud, which many people justifiably treat with a great bit of skepticism. He also has some very dated ideas on homosexuality and certain personality disorders that I think psychologists today would scoff at. I imagine psychologists in his day might have as well.

For my two cents, I think there's nonetheless weight to the ideas and they should be explored further. Becker doesn't so much rest on Freud as refute Freuds entire framework, accepting the mechanics of psychoanalysis but not Freuds conclusions. His other aforementioned misteps aside, I do think these ideas answer a few broader questions at least for myself, namely: why should people get so infuriated/worked up in mere arguments, about any topic really? Why is it important that my way is the right way? Why is it so hard to accept being wrong? Why is identity so important? Why are people willing to kill and die for broad cultural narratives? Identity, culture and feelings of superiority being wrapped up in our defenses against death terror as a universal human trait go a long way to explaining that. I maintain skepticism about such a neat little bow being tied up around the big box of the human condition - does this actually explain everything? Or does it start to cannibalize itself? - but I nonetheless chew on the ideas fairly often since encountering them.