People have some expectations of privacy and it's not normally considered acceptable to violate this.
Sometimes this stuff is untried in court or falls into a definite legal grey area and usually the policy is to err on the side of caution and simply assume that if something is commonly expected to be private, then it's private and should be kept so.
If we were investigating a user for XYZ and came across a file named "Personal Diary 2019.txt" or whatever, I can assure you that HR would not want us to open that file. Possibly if HR found out they'd declare the investigation tainted and want to stop it right there.