Answer:Because those emails are not private. They are Company property. They may well be required to be divulged to other team members, company officers and even law enforcement and so on.
Not least - the email administrator can read all of them.
There is no expectation of privacy with respect to email that you read on write on the Company’s behalf.
…and it is written evidence.
And if your Company has outsourced its mail to Office 365, say, then Administrators have tools you would not believe to search through it and find anything they want.
The best way for a wayward remark or inadvisable statement to not be found by Compliance officers is for it not to exist in the first place,