I used to write those letters when I worked in insurance. They had to be reviewed by legal, needed to make it clear what level of threat was involved without divulging certain kinds of info and only occurred when an actual breach of some sort had happened.

In my case, it was usually not a computer issue. It was usually a case of "We sent a check or letter to the wrong address" and it was weirdly common for the reason to be "Because your dad, brother or cousin with a similar name and address also has a policy with us and you people are nigh impossible to tell apart."

And we couldn't say anything like that.

Point being that divulging the issue comes with risks of making the problem worse. So it's not as simple and straight forward as it seems.