Unlike much of government, there are no hard requirements on the sort of people who can be hired to be program managers. This is important because The best DARPA program managers are the ones who can look at an entire literature in an area and notice a systemic bias, an attribute that is likely to make them clash with established structures and thus not be as heavily credentialed.
It’s also important because the bar for program managers is very high and the compensation is pretty low for their level of skill (DARPA Employees aren’t paid very much compared to what they could be and Why do people become DARPA Program managers?) so it’s just straight up hard to find people who would both be good at and willing to do the job. The only flexibility you have in that situation is on credentials and profile.
Bureaucracies ensure consistency through rules, so there are no safeguards against getting a shitty program manager. The fact that there are no guard rails makes DARPA extremely dependent on the A players hire A players and B players hire C players principle. It’s a double edged sword because one reason hiring rules exist is that people tend to hire people like themselves, which leads to inbreeding which DARPA has been subject to in the past.