Because the story sounds like something out of the fevered dream of someone with a loose grip on reality. Also the SS also is saying it never happened.
This makes her story even less believable. They interviewed the alleged source of the story, and it didn't come up?
ETA: It also seems obvious that with all of the witnesses they spoke to, someone would have mentioned this story before now.
I agree that Trump's behavior sounds like someone with "a loose grip on reality", but reports of similar erratic and violent behaviors have been reported about him in the past, so doesn't seem incredible.
CH relates what she heard, in an office where only Ornato and Engel were present. The story Ornato told her occurred in the presidential limo ("the beast") where Ornato and Engel were with the president.
Witnesses answer questions they are asked. If the committee didn't ask about this particular episode, Ornato and Engel may not have felt the need to bring it up. They are invited to return to the committee to rebut.
Here's Hutchinson's description of what occurred after Ornato ushered her into his office.
CASSIDY HUTCHINSON:....When I went in, he shut the door, and I noticed Bobby Engel, who was the head of Mr. Trump's security detail, sitting in a chair, just looking somewhat discombobulated and a little lost.
I looked at Tony and he had said, did you f'ing hear what happened in the beast? I said, no, Tony, I — I just got back. What happened? Tony proceeded to tell me that when the president got in the beast, he was under the impression from Mr. Meadows that the off the record movement to the Capitol was still possible and likely to happen, but that Bobby had more information.
So, once the president had gotten into the vehicle with Bobby, he thought that they were going up to the Capitol. And when Bobby had relayed to him we're not, we don't have the assets to do it, it's not secure, we're going back to the West Wing, the president had a very strong, a very angry response to that.
Tony described him as being irate. The president said something to the effect of I'm the f'ing president, take me up to the Capitol now, to which Bobby responded, sir, we have to go back to the West Wing. The president reached up towards the front of the vehicle to grab at the steering wheel. Mr. Engel grabbed his arm, said, sir, you need to take your hand off the steering wheel.
We're going back to the West Wing. We're not going to the Capitol. Mr. Trump then used his free hand to lunge towards Bobby Engel. And Mr. — when Mr. Ornato had recounted this story to me, he had motioned towards his clavicles.
LIZ CHENEY: And was Mr. Engel in the room as Mr. Ornato told you this story?
CASSIDY HUTCHINSON: He was.
LIZ CHENEY: Did Mr. Engel correct or disagree with any part of this story from Mr. Ornato?
CASSIDY HUTCHINSON: Mr. Engel did not correct or disagree with any part of the story.
LIZ CHENEY: Did Mr. Engel or Mr. Ornato ever after that tell you that what Mr. Ornato had just said was untrue?
CASSIDY HUTCHINSON: Neither Mr. Ornato nor Mr. Engel told me ever that it was untrue.