Why would a recipient of such an email, assuming it even makes it by their spam filters, assume it to be real?

In particular, wording like this just seems like spammer nonsense: "must depart the U.S. immediately... there is no grace period ...We advise you to depart the US as soon as possible"

Whether or not this is one real--it certainly looks fake to me--I predict an imminent flood of "self deport" flavored spams.