Part of it is because Mods don't want to decide what is or what isn't a scam (apart from the obvious cases there are many gray areas), and part of it is because having Mods delete scams will create a fake sense of security (Mods can't stop all scams).
So, unfortunately, we'll be stuck with scams.
Yeah! A mean, how could mods be so certain of what project is a scam and which is not! There is only so much as the forum could do. The forum has done well to input a trust system and even the red flags on suspicious projects for obvious reasons of a scam possibility. What else do users need...?
When I saw this thread, I was eager to see the impossible reasons as to how scamming can be halt and I wasn't disappointed to have seen non because, it isn't possible.
Just as LoyceV stated having mods decide would create a sense of fake security as people will be more open than ever and scammers would be sure to make use of this unalertness.
Its very unfortunate but, scamming is part of the system.