Buy at 99 something worth 100 and your edge is 1.
Positive edge is necessary but not sufficient. Three reasons a positive-EV trade can still be wrong: the size may risk ruin; your edge is an estimate with error, and edges computed from small samples are especially suspect; and adverse selection means realised edge is systematically worse than calculated edge.
The useful comparison is edge per unit of risk, not raw edge - which is why a market maker prefers thousands of small edges to a few large bets.