So it works like this:
- If you detect some wrongdoing, like somebody attesting twice, you can get a reward for whistleblowing (free money!)
- However to claim this reward, you have to include it in a block. Therefore if everybody is aware of this wrongdoing, the next block will get the rewards for slashing/whistleblowing.
- Therefore it's economically advantageous to keep this info to yourself, until you get to propose a block, and only then slash and get the rewards.
- There are also what are called altruistic whistleblowers, that as soon as they find wrongdoing, they'll broadcast it (instead of keeping it to themselves) so next block proposer will slash the wrongdoer. These kind of whistleblowers will most likely be run by the Ethereum Foundation
- In practice even if there were no altruistic whistleblowers, it wouldn't really matter because the same software that validators run can also detect wrongdoing, so in practice even with no altruistic whistleblowers, the next block is almost guaranteed to slash the wrongdoer anyway (free money!).