Denial is what caused this horror story.

This pod cast does a good job explaining Tapper and Thompson's research into their book. They interviewed over 60 WH staff and Cabinet members about Biden's decline and discovered how a small group of insiders insulated Biden and convinced him to run for reelection, with such catastrophic consequences.

Tapper had an interesting anecdote from the Biden-Trump debate which he and Dana Bash moderated. When Biden had that disastrous answer to the second question, Dana Bash slipped him a handwritten note that said "He just lost the election."