First of all, the police and the military are special cases because they are taught to work as a team and protect each other’s very lives. They all go through a lot of hair-raising stuff together. So their situation is a little different. Let’s leave them out.
Second, there is a difference between “reporting your colleague” and “reporting your colleague for child sexual abuse.” The latter is so awful that in the past, before these sorts of stories became widespread, a lot of older people just could not believe somebody they knew and liked did such a thing. They figured it had to be a mistake or a really bad error of judgment or even that it was somehow the kid’s fault. I have seen this in senior citizens who just could not believe that somebody they knew for years would do anything so awful. As someone else said they dismiss it as a misunderstanding or a situation that just got out of control momentarily, etc.
Third, these priests probably did a lot of good things in the daytime - they baptized babies, administered the Sacraments, said Mass, visited dying people, performed weddings, counseled and helped people. It is really hard for most people to wrap their minds around the fact that somebody can be doing good things and being a nice person and yet have these dark sides and terrible secrets. I saw some priest giving an interview about the notorious sex abuser John Geoghan and this priest talking about him was calling him a “living saint” and all this other complimentary stuff. Good and evil can exist in the same person simultaneously. And before we all realized that pedophiles don’t somehow recover after a couple months of rehab, it was likely thought that the basically good priest could get over it and not do it again if he just prayed and exerted some willpower.
I am also sure there were priests who didn’t report because they just didn’t want to get involved in some big mess that might interfere with their continued career as a priest. The priests in the report who said, “Don’t tell me the name” when victims would come in with tales of abuse - those priests just didn’t want to be involved.