It is quite possible for someone to take over someone else's account, and the last time I checked user deletion is a non-reversible process, by which I mean that it is not a single button undo, but requires a non-trivial amount of work to undo.
For instance, were I to sit in a cafe near Jon Skeet, sniffing the packets as he answered questions on stack overflow, I could readily cause actions on his behalf on the site. This is because there is only a limited amount of security on the site.
It is also one of the reasons why high reputation, trusted users are still subject to bot detection algorithms.
Email confirmation would be great, except that once you're spoofing another user you can readily change their email. They could change the system and require email confirmation of email changes, etc, and lock things down further, but at the moment they don't, and there's a good chance that if I'm sitting at a cafe next to another stackoverflow user, it's likely that I've got enough information to deal with trivial email problems as well.
Account deletion is not something to be done lightly, and so even if we overcame all the security issues, stackoverflow still has a need to understand why users want their accounts deleted, and how far that extends into site content - in some very rare cases it's reasonable to delete the user's questions and answers, but in most cases it's not. The team needs to understand the situation before such actions are taken.