Also, many have been in operation since before AWS was a thing, and some are larger. So I can't imagine what AWS knows about running a datacenter that others don't.
Now maybe in theory if you can build something to be fully one with the cloud, considering all edge cases, and limiting yourself to only cloud-zen tools (or building your own, or doing vendor lock in). In theory, with enough money, I guess the cloud lets you maybe achieve higher reliability.
The fundamentals of EC2 (lack of dual NICs, dual power supplies and BBU RAID + virtualization and general complexity) means that a single instance is way less reliable (let alone, much worse value) than a single dedicated box. The complexity you need to throw on top of that building block (in the shape of lock-in, compromise, money, latency, application complexity or a combination of these) is pretty significant.