I don't really think this is odd: they have been asked to send the assignment by email and they did. There isn't really a need to say anything further and they didn't. Maybe it would be slightly more courteous if they were to add a couple of words of greeting, but your job is not to be Emily Post.
If the assignments were submitted on paper, and they left their assignment in your mailbox, would you insist that they include a note wishing you a nice day? No, that would be silly.
There isn't anything you need to do about this.