Because he has chosen to save Casey and lie instead of properly grieving the loss of his grandaughter. It is a conflict for the natural process of grief over death and loss. It will create alot of depression and suicidal thought because he is "suspended" in this path he chose, where he cannot move forward in healing himself from the loss.
He is not stuck in the "denial" phase of normal grieving. There are 5 normal stages...shock/denial, anger, depression, bargaining, and acceptance. The A's have added a phase after denial...called "we know she is dead, but we have to save Casey" which really screws them emotionally...they cannot go to the next phases of the healing process. It is on hold for now.
Denial does not last this long. They are clearly past that. These are ANGRY people. They are stuck in that phase. We tend to really dislike them because they are so angry and hostile (and also because they are lying)...They will self-destruct if they don't move on to the next phases of the grieving process. Such as evidenced by George's "suicidal ideations", and Cindy's obvious self over-medicating. The anger is making them more delusional every day. The dam will burst one way or another..They will come to tell the truth and move on, or they will self-destruct.
So well said. It's like they've deliberately chosen to create and exist in a fantasy which makes it impossible to move forward in the real world. Can't be good for their long time prospects, or mental health.