Because of market dynamics. Serious arbitrators that want to do arbitrage on the long run as a "living" won't sale their reputations in the first place. Of course in a small pool it would still be fairly easy for a scammer to buy enough accounts but impossible in a large pool.
I don't think it is realistic to say that you could earn a living acting as an escrow agent in this regard. This is especially true considering that your reputation is at risk anytime you make a decision as it could appear that you are scamming (when you are really not). I have spoken to a couple of people who use escrow services on this forum and have been advised that they generally would only have a few hours worth of work from providing escrow services each week. If this is true, then in order to make providing escrow on open brazzar a "full time job" then they would (ironically) need to have (either by buying accounts or creating them) multiple accounts