It's a personal belief. If they trade for whatever reason and scam when the opportunity presents, they have two reasons to stay. If we take away the frequent successful trades, it can make it not worth their time to stick around purely to scam.
Rephrased, if the site is more functional for a person, a person is more likely to hang around. The purpose of this thread isn't to solve problems though, I am new and can only pretend to know the ins and outs of this forum. Case in point, the fact that this thread exists.
There isn't really a great way on this forum to know someone is a scammer if they are here doing successful trades so I'm not sure how they could be identified before they decide to scam. Also keep in mind that "successful trades" can be faked by other accounts owned by the same person (alts), and they can leave as much "untrusted" feedback on themselves as they want, making it look like they are trading with other "users" when they really are not.
Members with "green" trust on their trusted 0/0/0 score are usually the ones with actual proven trades because they have been given positive feedback by other DT members, whom are generally trusted by most of the community, but my belief is even green users should also always be looked at before trading. You should always look at the user who is leaving them feedback as well to ensure it's not just random newbies/alts or other scammers themselves posting feedback for whatever reason.
Once a scammer's account has been marked "red" by a DT member, their accounts usually become pretty worthless to them because nobody (except maybe other scammers) are going to want to trade with them, and they cannot easily sell their account or use them in most signature campaigns.
The escrow's job is to act as a trusted middle man between the trade to prevent the opportunity to scam. If an escrow was always used, there should be minimal opportunity to scam. There is a chance the escrow turns into a scammer themselves, but this is much more unlikely, and rare, especially if you are using a reputable escrow that has handled a lot of successful trades with large amounts and has been around a long time.
The admins could probably do more to proactively fight scammers but I believe they have deemed it as just too much overhead to monitor and really just put the burden on the community members themselves to be vigilant.