Because they all were involved. No SINGLE entity organized, arranged, and executed this.
They all did something little, something they could justify, something that was just a block in a LEGO construction.
Change in personnel. Change in financing for different departments. Letting something questionable get through on a legal document. Having a blind eye to certain 'visitors'. Holding up a bloodied caricature of Donald Trump's head. Stating that someone needed to put a bullet in Trump. Not showing up for work the day of a rally you are involved in. Trying to keep the mystique of your agency above others by not having real time communication. Showing up to broadcast a rally when you've avoided them previously. Setting your camera to the highest shutter speed so, JUST IN CASE, someone fires a bullet, you might capture the image.
Everyone did something or wished something and the coverup is so they can all hide what they did.
They believe, and want you to believe that they are impartial and honest. Because it wasn't an organized perfect game plan run through channels, no one can specifically be blamed.
But they agreed with the idea and therefore contributed where needed.
Yup ... some often referred to examples of probable foreknowledge:
July 2, 2024: Nancy Pelosi speaks to MSNBC's Andrea Mitchell
"This is not a normal election where you want to win and if you don't, you cooperate and do the best you can for the country and hope to win the next time. This is something that is undermining our democracy. He must be stopped. He cannot be president!"
July 5 2024: Stephanopolis & Biden Interview:
S: ... it's gonna be much tougher to win, in 2024?
B: ... Not when you're runnin' against a pathological liar. Not when he hadn't been challenged, in a way that he's about to be challenged.