We must face reality. Trump is not a stupid man. Far from it.

[Kaleva]

30. Trump's transition efforts were sloppy

"The Trump transition has been assessed by some as having been a troubled transition, with many calling it "chaotic". The chaotic nature of the transition has been largely attributed to Trump's decision making, including his firing of Chris Christie and scrapping of the pre-election planning which Christie had undertaken for the transition, which meant that the transition had to start their planning from scratch.[94][95][96][97]

In 2020, Tom Wheeler wrote for the Brookings Institution that "the Trump presidential transition in 2016-17 proved that a transition delayed is a transition denied", arguing that the Trump transition's delay in placing transition teams at federal agencies was ultimately damaging to the Trump administration.[98]"

https://en.wikipedia.org/wiki/Presidential_transition_of_Donald_Trump

"Christie: “He became convinced that he had to do this legally and he just tried to stay as far away from it as much as he could. The only time I ever really spoke to him about it…was on rare occasions where he read something in the news about the transition and he would then call and give me some reaction to it. Each time he called, he’d say to me, ‘Chris, you’re wasting a lot of time on this. You and I are both so smart, if we win this thing, we can do the entire transition if we just leave the victory party two hours early.’” "

https://presidentialtransition.org/blog/chris-christie-on-trumps-transition-to-power/

A couple of things I takeaway from the above is that Trump wasn't sure he'd win. The second is that he thought his chances of winning were low enough that the transition team didn't need to make much of an effort preparing or he had deluded himself into thinking that he was so smart that if he did win, the transition would be a breeze.