They feel guilty for having survived so they pretend the bad things never happened
Exodus (1960) screenplay

In order to believe clients' accounts of trauma, you need to suspend any pre-conceived notions that you have about what is possible and impossible in human experience. As simple as they may sound, it may be difficult to do so.

Many survivors struggle to believe the abuse happened. They don’t want to believe it. It’s too painful to think about. They don’t want to accuse family members or face the terrible loss involved in realizing “a loved one” hurt them; they don’t want to rock the boat.

Being in a state of denial is a
universally human response to
situations which threaten to
overwhelm. People who were abused
as children sometimes carry their
denial like precious cargo without a
port of destination. It enabled us to
survive our childhood experiences, and often we still live in survival mode decades beyond the actual abuse. We protect ourselves to excess because we learned abruptly and painfully that no one else would.

As you may already know, post-traumatic stress disorder is extremely complex. Each client has a unique, perhaps virtually unbelievable, set of experiences, and an almost equally set of reactions to those experiences.

Denial forces victims to retreat in lifeless existence, dieing in the shadows of buried trauma and painful memories.