Caching proxies are old hat. Things like squid have existed pretty much the entire lifespan of the web. Keeping things close to your servers means you don't have the inherent delay of pulling things from a remote server, regardless of how fast amazon makes things.

Additionally, not keeping all your eggs in Amazon's basket means you're not SOL when they have a datacenter hosting all your content go down. It also means that if and when a service that better fits your needs comes along, you are more readily able to migrate without problems.

Finally, site reliability is not something that takes a team the size of Amazon's. A lot of the things that are required for availability on AWS -- redundant systems providing services, standbys, etc -- are things that sysadmins were doing before AWS was extant. AWS' biggest gift to reliability is that its instances are less stable than most dedicated servers; you're taught from day one not to rely on a single server, so you build it right the first time.

So no, it's not hubris. It's calculating price/performance, it's applying things you're probably already doing to a new problem, and figuring out what the best solution really is, which rarely involves just throwing money at Amazon.