I've pondered this too.
~Organized rides are a great place to meet people and make new friends.
~Friends are highly prone to flaking on informal rides.
~Riding with long-time friends is boring. It's too safe when everyone knows everyone else's foibles. Much more adventurous to ride with a pack of wobbly, Parkinson's afflicted newby-triathletes who can't tell the difference from a red or a green light.
~The only medal you get for an informal ride is the $#it you put up on Garminconnect. Organized rides give us promotional items such T-shirts, bottles, patches, which can be prominently displayed to reflect one's scab-defying feats in the Valley of the Wobblies.