Who takes the risk in each scenario is different. If Amazon was only using their own sales data then they'd need to start selling a more diverse range of products themselves to acquire that data. Some of those products would sell well and some would be flops. Amazon would have invested money into selling all of them so the cost for the flops negatively affects their bottom line.
In the current scenario someone else sells things on Amazon and is taking the risk that the item they're selling will not sell well. If the item is a hit, Amazon swoops in and starts selling it themselves or possibly makes a competitor and sells it themselves. Either way, Amazon reduces their own risk of selling poorly performing products while also cutting into the upside for the vendor who took that risk when they are successful.
Edit: I forgot to mention above that the people taking this risk are paying Amazon to do so.
In the current scenario someone else sells things on Amazon and is taking the risk that the item they're selling will not sell well. If the item is a hit, Amazon swoops in and starts selling it themselves or possibly makes a competitor and sells it themselves. Either way, Amazon reduces their own risk of selling poorly performing products while also cutting into the upside for the vendor who took that risk when they are successful.
Edit: I forgot to mention above that the people taking this risk are paying Amazon to do so.