Disclaimer: I worked on an AWS service team.
This is, oddly enough, similar to a debate people have about consumers TV or Internet: should pricing be "unlimited" or "a la carte"?
AWS is combining all your networking charges into one lump "outgoing data transfer" fee. So it's heavily marked up in comparison to what they're paying for the outgoing data transfer, and you're not sure how much is profit vs. whether it's going to cover all their other costs.
So it might be fairer if AWS broke out separate line items for internal, incoming and outgoing data transfer, plus all the additional systems a customer uses.
I think AWS's billing is probably already on the falling side of diminishing marginal returns. That is, it's complex enough that more information would tend to hinder customers from getting the best price. Right now, if I plan to reduce my data charges, I have one variable to tinker with. If we expand this, it would mean I'm having to balance incoming / internal and outgoing charges. That sounds simple, but in terms of engineering it can be very complex.
The next claim is that this biases customers not to move. Of course, Azure and GCP have the same arrangement, so while you pay to move out of AWS, you don't pay to move in to Azure or GCP. So all the vendors are attempting to lock you in to their product, and at the same time trying to extricate you from their competitors, overall it's a wash.
So, yes, part of the motivation for egress charges is that ingress is a loss leader. But it's also true that egress is a metric that does, for the vast majority of their customers, directly translate into customer value. If there's a compelling case for doing it differently, someone should do it and see if it works.