There are a lot of resellers of various services with different levels of value-add. The reality of the hosting business is that different providers give different levels of service, and in a lot of industries you do not want your customers to know your costs, so breaking it out as a separate line item isn't always optimal. Businesses actively try to avoid their product being viewed as a commodity.

There are also purchasing people in big companies who are paid to negotiate down quotes. If you tell them there is only one price for EC2, and they can't get a 5% discount for a large order, they will in some cases have a perverse incentive to go to a more expensive (list price) provider who will give them a discount (so they appear to be doing their job), even if the total price ends up higher, or the service ends up worse. This is one of the many problems with enterprises.

I'm not saying which model is better, but it's definitely a factor with some clients.