Because it creates a positive feedback loop. More police means more incidences of law breaking get detected as crimes, which results in more police enforcement.

Imagine a road network and police with a limited number of speed traps. If you send the speed to the places where there have been more people caught speeding then the fact that the speed traps where in that location will have a higher rate of caught speeders.

Another example is drug enforcement. Black people are searched more often then white people for drugs, which results in them being caught having drugs more often which in turn makes police think that they are more likely to have drugs.