Doing that suggests they think that the threat from insiders >>> outsiders.

It always is. Insiders have more access. They are trusted. That makes them a much greater threat if they decide to do harm.

I think that ... certain nations ... are quite good at playing "the long game," and often have their plants hired by companies; sometimes at fairly senior/sensitive levels. They may not even start phoning home, until they have had a few promotions.

Well when the concern is leaked internal memos, that is a very reasonable assumption.

I think such a company would equally want to not retain such records (at least not indefinitely), for fear of discovery in legal processes, etc.

Or most likely laziness.

Or a fortuitous combination of the two...