Why aren't they making a dlc like this

First off, you're new to the boards so I'll play nice. I've been known to get, animated, when this topic is posted because I'm very well against the idea of TES having online content.

Discussions about co-op/multiplayer has a thread of it's own over here: http://www.gamesas.com/topic/1320106-official-tes-multiplayerco-op-thread/

Best to take your reasons to why you feel this series should be online over there, my friend.

because they aren't

Because these people say so.

Modders are working on something like this, i'm sure.

There will be no official multiplayer/co-op for Skyrim. If you wish to discuss the possibilities for future ES games, please do so in this topic -

http://www.gamesas.com/topic/1320106-official-tes-multiplayerco-op-thread/