Not every decision is about cost minimisation. There's a resiliency motivation to place DCs in separate flood plains and separate power grids if possible. Whether AWS is following that, I cannot say.
"Each availability zone is designed as an independent failure zone. This means that availability zones are physically separated within a typical metropolitan region and are located in lower risk flood plains (specific flood zone categorization varies by Region). In addition to discrete uninterruptable power supply (UPS) and onsite backup generation facilities, they are each fed via different grids from independent utilities to further reduce single points of failure. Availability zones are all redundantly connected to multiple tier-1 transit providers."
[1] https://d0.awsstatic.com/whitepapers/aws-security-whitepaper...
Does inter-AZ traffic traverse only fiber that you wholly own? Does AWS send traffic over some shared links in some cases? If only some cases, which cases?
Of course later on the IRA bombed the building next door to us - we survived ok the modems just rest them selves