Because you can then reallocate the funding from the solution that doesn't work (militarised police, mass incarceration) to solutions that do. The police don't prevent people from firing thirty random shots into a house, but decent alternatives to crime - even for those with little or no educational achievements - might. It requires a holistic approach though, with multiple agencies and policies working together, which is a hard transition from targeted intervention, whether that be policing or sticking plasters.

There's a leap of faith involved in making that connection, but it is a small leap once you look at other countries and see how their systems work. The fundamental step is to not make life so awful that desperate people decide a high-stakes life of crime is worth it.