Continuity mistake: When Frank is arriving at the school to pick up Jack at the beginning of the movie it shows him pulling up to the school and there are no other cars parked on that side of the street. When it cuts to show him sitting in the car waiting, there is a car parked behind him. When it cuts to outside again, there is no car behind him.
Continuity mistake: In the car chase after the doctor's, the bomb on the bottom of the car stands out while Frank drives away, but in the chase where you get a lot of low angles the bomb is very obviously gone. Then it appears again when leaving the boat yard / docks ready for the crane scene.
Continuity mistake: During the escape from the police the Audi's headlights switch between being on and off repeatedly.
Continuity mistake: When Frank is on the Jet Ski with the female, chasing after Dimitri, the chipped white paint on the Jet Ski's front hull suddenly has a perfectly painted surface in a couple of close-ups. (00:46:30)
Continuity mistake: When Frank finds Dimitri at his residence, Dimitri is wearing two bracelets - a chunky one (closer to his hand) and a thinner link - on his right wrist. Either both disappear and reappear or one disappears and reappears or they switch positions, such as when he is hanging onto the back of the moving bus. The pinky ring on his left hand also performs a vanishing act. (00:44:25)
Continuity mistake: Frank goes back to building where the pediatrician's office is located, finds the security camera video and snaps the overhead photo of Dimitri looking over his left shoulder to face the camera. When Frank sends the photo to Inspector Tarconi, who is at the US Marshal offices, the photo of Dimitri is entirely different. (00:39:25 - 00:43:05)
Answer: The police will just get in the way and slow things down too much. Frank can operate outside of the law and get more done and get it done quicker. Plus, they can't just take Audrey's word for it.
TedStixon