That's because every time the police force would call a potential victim to warn them, and it would turn out to be a false positive eventually some business owner would think: "we lost some money today, must have been because of that false warning, police is do incompetent".
And then proceed to take the force to the court for "damages".
I don't believe for a second that the primary reason police departments don't look for potential victims is because they're worried about getting sued. The way they handle police brutality cases is enough proof that that's not true.
And then proceed to take the force to the court for "damages".