Sometimes they do.

They seriously botched the Branch Davidian raids twice by underestimating the evil of David Koresh and his minions. And that's how it usually is with the FBI and most other law enforcement agencies. They get complacent. They think too well of people, and expect them to act civilized, like the vast majority of people they deal with. After several thousand raids, undercover stings and arrests, they get the notion that nobody will really fight back like homicidal maniacs.