They can't be sure of the profits because they have to wait 5 minutes, and their 5-min price prediction will carry substantial risk (as former top quants, we know for sure that nobody in the world can continually make 5-min predictions with more than 70% certainty). There is a huge risk to price prediction, so their rational choice is go to some other exchanges to execute their near 100% sure-profit trades, instead of taking their, at best, 70% chances with us.