It's much more complicated to find out if something is a scam. You have to question various people, gather evidence, listen to the accused and then do a verdict on whether he's guilty. It also leaves a lot of room for opinion and bias. Someone of the mods could be affiliated with the user suspected of scamming and would ignore certain evidence. Aside form that, moderation of scams gives a false sense of security. Let's not forget that with so many scams, moderation of those would practically be the only thing we'd be able to do.