When you work for a company, all email communications are the property of
the company.
However, companies are also sensitive to "personal privacy" issues.
Your comment is very logical.
But, all it takes is some guy in the computer dept saying,
"Oh, Matt is no longer here. Good, I can elliminate his email address."
I do think many companies forward the former employees email to
the person's secretary/administrative Asst. And, that person manages
the acct temporarily.
Companies do not want to "drop the ball" because someone has left the company.
It would be interesting to hear other peoples experience with this type of situation.