There may be other reasons, but trying to look like you’re doing a good job can be a conflict, as while higher crime rates might justify higher budgets in the future, they’d also lower the rate of solving and preventing crimes which are seen as metrics of police success.

Other than relying on 100% of police to be 100% ethical (and even without congnitive biases), which obviously isn’t ever going to be the case, third parties are probably always going to be necessary to improve accuracy.