Caveat emptor: I'm not a professional coder, so I could be completely wrong.
It's not necessarily that multiplayer support would require difficult coding to implement, but the logistics behind it are a lot more difficult. Adding networkable multiplayers introduces issues that transcend mere code. How are you going to prevent cheaters? How are you going to deal with latency issues?
I think most people will find it time-consuming enough to create a standalone rpg, but to have to create a world that is constantly evolving... that requires resources that the average reader of this webboard simply does not have available to them. I'm sorry if this comes off sounding elitist, but you're going to have to find a dedicated server with a lot of bandwidth (no, your cable modem isn't enough), have some really solid server software, have 'admins' of the world, as well as 'creators,' since I sincerely doubt one author can keep the multitudes entertained for long.
If you really want to create your own online RPG, go start a MUD. People still use those. They require a LOT fewer resources and are concievably something that your average person could run off a spare machine on their cable modem.
Perhaps in ten years, the tools to let hobbyists create online rpgs will be available. I don't see it happening anytime soon, however.
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People who claim the sky is falling obviously aren't aware the earth is falling, too.