Mostly because only like .5% of people use it, and there are already so many free ways to do it for most games the time an money isn't worth it to make it. Brad suggests to just set up a Dropbox with the save directory with a friend and play that way. Same thing. They are only a small company with like 20 people, they prefer to use that money elsewhere, which I kinda agree. That being said they might look into it later down the line like after an expansion if they have the time an people really want it.
I would also love play be email.
How does the save/restore session work in multiplayer.
Do both human players have to be online?
Can I create a session make me move save the session email the file to my buddy and then he moves (like in Civ 5)?