[EvilTerran]

Some reasons would be:

* Every field you add to a form reduces the number of people who complete it -- as it gets longer, more people will go "ugh, big form, not worth it" and click away the moment they see it.

* Autocomplete usually provides my email address for me, so asking twice is pointless; when it doesn't, I usually copy-and-paste from one email box to the other, and I'm not alone in doing so (see elsewhere in these comments).

* "These email addresses don't match" leaves the user to play hunt-the-typo hirself, while the submission's approach points hir straight at the (possible) error. I guess you could highlight the differences with the double-entry approach too, but suggesting plausible corrections remains easier for the user.