Basically because it was the spirit of the time (~2002) when XEP-45 aka XMPP MUC (groupchat) was defined. XMPP MUCs immitate mostly IRC with a few more features. It's not suprisingly, as this was the chat protocol that dominated that time period.
More then 10 years later we know better. First the Publish-subscribe pattern became popular, and was specified for XMPP in XEP-60. We now know that this would be an ideal candidate to implement (group)chats that can be easily synchronized between multiple JID resources (devices of a user), and where 1:1 chats are just groupchats with 2 participants.
The only open implementation I am aware of, that comes close to such a design and also provides some drafts regarding the specification, is provided by Buddycloud: http://buddycloud.github.io/buddycloud-xep/
I recommend looking at their open-source software. The project exists for a while and seems be mature for production use: buddycloud.com