I updated my reply—please see the above links.

Again, these egress fees are not paying for egress alone. They are defraying the cost of operating a massive, complicated, low-latency, reliable cloud network infrastructure, much of which is free of charge when used internally.

Some, like you, are expecting that cloud providers operate on a “cost plus” model where what you pay is strictly based on what marginal costs for the same thing. But it’s not that simple in reality. Charges levied for one thing can be used to pay for another.