You also need to house these servers, including backup power etc., manage them, maintain them, develop and deploy the software etc.. Also, you should really check the power usage of enterprise disks and servers; it may be cheap, but it's far from comparable to your average desktop. Then you need to add in that they need to have a reserve capacity as well; you can now go and store 100TB on S3 and AWS will be fine with it - but they need to have those disks up & running already.

Don't get me wrong, S3 is expensive, but replicating the availability, feature set and scalability is going to be very expensive, too. You can cheap out if you don't need these features, of course.