'Hey yourself'

'hello'

Or any other similar mannered greeting really.

People who write a single greeting consider chatting to be a 'written conversation' and are basically following the rules for spoken conversation, where you first greet the other, wait for a greeting back as an acknowledgement of approach and offer to converse. Usually followed by an introduction and whatnot.

In chat these steps might feel redundant as the fact that there is a connection between you two already implies an eagerness for the conversation, and an introduction is not necessary thanks to nicknames and aliases, but for people who rarely chat, the norms and rules of chatting might be unknown or feel alien and discomfortable. Doesn't mean they are bad conversation partners necessarily, just that they are a bit awkward when it comes for chatting.