Corruption. Lack of evidence on some other cases. Personal revenge. Who knows, but list is big.
Ok but but what ends?
Imagine you’re a journalist uncovering corruption perpetrated by the police force or a politician. Can you see how they would be incentivised to arrest you on false charges to halt the investigation and protect themselves?
performance metrics and careers exists in law enforcement too.
Goodluck getting a promotion if you never arrest anyone. I know some cops who plant weed almost every spring, just to "bust" it every fall.
This is a pretty weird question considering the mountains of documentation of authorities doing just that. This is not some kind of hypothetical that needs extraordinary justification.