They are operating their payment gateways for business that actually have operational cost so they have to balance that cost with income, to maximize their profit.

Those services can be the same or different at different payment gateways as well as different blockchains. They have to proceed on-chain transactions and each blockchain has different native currency for transaction fee. Ethereum blockchain has most expensive transaction fee because of high demand on that chain, its bad design and high price of Ethereum $ETH.

However it won't stop people using Ethereum ERC-20 chain for transactions. Last but not least, they have options to choose, from blockchains to payment gateways and you are free to make their choices.