Police departments have major limitations on manpower and budgets. Setting up sting operations with fake packages, surveillance, and all this stuff is going to be extremely expensive on both accounts. And the ultimate reward is that you just busted a guy who either committed a very minor felony which some states, such as California, are even moving to change to a misdemeanor. And I'd imagine a decent number of the offenders are not exactly financially well to do, meaning they may not even be able to pay the fines to recoup some of the costs involved in busting them.

Think about the sort of petty offenses that are more actively pursued. You'll almost always find they tend to be low execution cost with a high probability of receiving payment: traffic violations, prostitution stings, drug related stuff, etc.