Raiding has to come with a sufficient reason for those raided to believe the raiding party already has at least a portion of the material, either obtained through interception or inside sources.

The idea is that the people being raided will hand over as much as possible because it’s in their own best interest - knowing that anything they don’t produce may be in the material already obtained, making them liable for destroying it.

A late and preannounced raid without any material obtained beforehand is useless.

(It should be noted that there is no need to actually obtain any material if you can make the people raided believe you have some of the material)

They may have deleted the data, but missed mentions of the data, or conversations that imply the use of such data, in internal emails, etc.

Ultimately, this is a company founded by Bannon, who’s nearly as good at hiring The Best People (TM) as Trump is (see Milo, for instance). These are not the world’s smartest people. Even if they did decide to indulge in a bit of illegal destruction of evidence, there is every chance they would fuck it up.

And destruction of evidence is probably not as common as people think in these cases; it’s a huge risk.