While I can understand the cynicism, the real answer is a lot closer to something much more boring, which is most people just don't care about the actual numbers, and if they were to release them, while interesting to a small few, generally no one would actually care.

There's also a common misnomer that Amazon.com is somehow just this one giant app running on a set of servers, which isn't remotely how it's actually deployed, and that's before we spend time arguing whether a team's instances even count as "primary e-commerce front-end stack" or not. :P