The GAS is legally a separate entity from the game companies (although controlled by them), and it sets a varying fee for its delivery network. Maybe the fee is quite high, and unlocking a $6 game feature actually consists of $5.50 fees paid to the GAS and $0.50 paid to the game developer.
Would Apple say: “Sure, this delivery fee you’re paying to GAS is something completely separate from the in-app purchase. We’ll just take our cut only from the $0.50 and let you pass all of the $5.50 through to this GAS corporation that you partially own.”
Of course not, because that structure would be such a transparent attempt by the game developers to evade Apple’s fee.
The situation in crypto isn’t all that different. These gas fees are ultimately paid to Ethereum stakeholders, and Coinbase happens to be a big one.