A corporate e-mail system is very different from personal e-mail. For example, you can't use a public-key cryptosystem in which only the user of an e-mail account has the key to that account. Corporate e-mail is the property of the company, and if an employee resigns, is fired or dies suddenly, the contents of their e-mail needs to be available so that their successor can carry on their business functions. (This is true for everyone from a customer service rep to the CEO.)
Also, e-mails need to be archived for legal reasons. For example, if the employees of a financial services company moved all their communications from e-mail to WhatsApp, the SEC would not be pleased at all.