Because it's not really detectable by patrolling without in-depth investigation. A patrol cop staring at e.g. a stockbroker doing insider trading would not deter it because that wouldn't be visible even as he would be doing the trades. A patrol cop sitting in all Madoff's meetings and sales presentations wouldn't prevent the Madoff scam from succeeding. There's nothing to suggest even getting a warrant without analysis of the actual trade data/accounting information/etc, which cannot really be done by most nonspecialized police officers.
Because nobody will make a study to verify that the sky is blue if there isn't at a minimum some dissenting opinion.