While your comment explains the origin of this culture... it's generally not as easy to change such a culture as it is to maintain it. Once a group of people believe that the rest of the world does not understand them or is against them, they are somewhat inoculated against efforts by the rest of the world to change them.
New officers spend most of their time with older officers, not department leadership, not the mayor, not journalists, not activists, etc. Older officers teach younger officers how to act, and to some extent, what to believe. And, they act collectively to punish new officers who fail to adhere to this culture.
This is well-enough known to become a storytelling trope: a young idealistic officer finds him- or herself facing not only criminals, but also the cultural inertia of the disillusioned existing police force, as they try to do the right thing.
In many contexts, we accept that organizations have to end, and be replaced, to enact meaningful change. Companies go out business; political administrations lose elections.
This is why the idea of "abolish the police" or "defund the police" might not be as crazy as it sounds on its surface. It's not that we don't need people who are paid to investigate crime and keep people safe... obviously we do. But police forces as they currently exist may have too much cultural inertia to evolve the way they need to.