Answer:
The fact that in a purely competitive industry there are neither barriers to entry nor barriers to exit is what makes economic profit, in this type of industry, impossible in the long-run.
This is because when there is economic profit in the industry, firms are lured to enter, saturating the market, and lowering economic profit to zero in the process. When this happens, some firms opt out of the industry, bringing economic profit back to positive territory, causing the cycle to repeat itself.
