I don't have too much knowledge about how the delivery of apps in iOS store works, but I could infer that it is not only hosting files. They need at least to keep multiple versions and delivery the one that fits the proper iOS version of the device. And, based on my experience with delivering desktop apps I think there is much more to that. For example I was just delivering one single app to customers and still I needed to allocate time to infrastructure and versioning and automating some tasks and signing the code and much more.

I imagine that for an iOS store there is more.