They are indoctrinated at the academy. It is drove into thier head.
Ask
@Aeroknow how cops are trained. If your one of the good ones that want to change things, your partners leave without backup in a dangerous situation.
The good ones quit or they shut up and conform real quick.
The judicial system is corrupt from top to bottom. Its all money. For profit prisons. Hell even state and county facilities are run like businesses.
There is no incentive to tell the truth and let you go.
They have quotas to meet. Don't meet them, look for another job.
Cops have a lot of incentive to lie. As already stated, they think its OK and part of thier job to lie.
At what point is one lie OK for an arrest or conviction and another one not?