Because the systemic risk is linked to very rare events, and with very rare events, no single actor will ever have enough data, at any rate pooling should help us get a better idea (still imperfect of course -there will always be the super rare event that all the data in the world will not help to evaluate, but let's have as much as we can at least).

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi