I think you completely misunderstood my point.

First, an AWS region isn't "a datacenter." It's several with very fast very expensive networks between. If you tried to replicate that, you might luck out and find two data centers a couple of miles apart that have such a network. If you do, they'll absolutely being charging you extra to use that bandwidth. Otherwise, you have to start laying fiber which isn't exactly a cheap endeavor.

So yes, what you might build might cost you 10% of what you would have paid aws, but that's because you didn't build something nearly as redundant or reliable. The point was most companies don't actually benefit from all that extra reliability and it isn't worth the investment. But you're paying for it in AWS whether you need it or not.