The correct answer is Economies of scale.

As AWS aggregates usage across a global customer base it can purchase hardware and network capacity more efficiently and spread fixed costs over many customers which reduces the provider's unit cost. Those savings come from purchasing power operational automation and accumulated experience and AWS can pass a portion of those lower costs to customers as lower prices over time.

Pay-as-you-go pricing describes how customers are billed based on consumption and it does not explain why the provider can lower list prices across the board over time.

Savings Plans give customers lower rates when they commit to usage and they are a purchasing option rather than the underlying reason AWS can reduce its standard prices.

Amazon EC2 Auto Scaling helps individual customers match capacity to demand and control their own costs but it does not create provider-level cost reductions that drive lower per-unit prices for all customers.

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