Mainly because group projects fail 90% of the time when there's no actual face-to-face communication. And getting random internet strangers to agree on an RPG story (let alone the battle system/items/weapons/etc.) is nearly impossible.
And, again, people don't like to work on "other people's" projects.
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This is why I decided not to go ahead and do one.
P.S. Me and my friend Liro are doing Axis Return together but we live next door to each other.We still fight over small things like item names though and it has led to a few physical tussles.