I'd wager that AWS's marginal cost for intra-region traffic is zero. They own their infrastructure and other than maintenance and upgrades they have no cost linked with traffic volume.
Some cloud providers charge a round zero for intra-region traffic, and they don't have anything resembling AWS's scale. I doubt it's a loss leader.
That's likely a selling point to attract people to the competition. "Look, our bandwidth is free!"
Also there is quite a lot of cost for internal, inter-zone, traffic. You need all the routing equipment and the fiber links for that. And such traffic does not go through the same links as the public one, because you'd be paying the cost of public bandwidth trough transits or peerings. You want your own backbone, but this has a cost that needs to be amortized. Furthermore, properly designing such a network has a cost in engineers.
OCI also doesn't charge for NAT gateways, which makes sense because it's a configuration in a router that already exists and is already doing things. AWS, on the other hand, brings out the knives for you.
They aren't and so they can't. AWS can and should be prevented from doing so.
But bandwidth is free in intra-node traffic, isn't it? I mean, in some scenarios this traffic doesn't even leave the server. What does localhost traffic cost you?
How much does Google pay for bandwidth? What percentage of the backbone do they own/lease/run?
System engineering is not measured in dollars per byte transfered.
(Also, whatever they pay, it is not charged in GB-increments.)
Some of the traffic doesn't leave the server, let alone the server rack.
I'd wager these costs have zero to do with operational or infrastructure costs, and are just a way to price gouge customers.