Probably because to get to a position where one can make these decisions, one needs a healthy self confidence, beating the odds is part of the day job.
On a serious note, a lot of failure are down to lack of homework. Buying for geographical expansion or purely to be the “biggest” usually fail, yet a lot of acquisitions focus on this. Sometimes, there is a pride element. The DD shows the acquisition shouldn’t go ahead, yet, decision makers do not want to go back on their words / announcement to the Board and therefore will try to see it through.