I found a DOJ study online a while back (its no longer available) that was focused on the use of force by law enforcement. It found that when LEO use force it is excessive in the majority of cases (around 70% if I recall correctly).
They also found that 57% of LEOs admitted to covering up illegal behavior of fellow officers. Thats admitted, not actual cases.
I’m sure the reason you stated is part of it. A culture of acceptance and ego are probably also part of it. They do it because they know they can get away with it.