A VM is too coarse grained and doesn't know how to deal with sensitive data in a structured and secure way. Everything's just in the same big box.
You don't want to give a a single agent access to your email, calendar, bank, and the internet, but you may want to give an agent access to your calendar and not the general internet; another access to your credit card but nothing else; and then be able to glue them together securely to buy plane tickets.