Also Quora answer: https://www.quora.com/Why-does-Netflix-use-Amazon-EC2
Too many companies roll their own solutions when they could take advantage of economies of scale like this and let another company dominate the problem.
Obviously there are privacy reasons to own your own data but I don't think they outweigh the performance and efficiency implications.
AWS is a tool, not a bullet made of silver.
Too many companies build their business completely reliant on a single service provider that can/will change focus, increase costs, terminate/change service offerings, or have unreliable service availability.
If you mean that AWS prices are too high, I think you're thinking in terms of retail prices. If you have enough volume, AWS will negotiate a volume discount.