Monopolies distort markets even when they act in a pro-consumer manner. For example the credit card companies. A basic credit card is really cheap and easy for the average person to use. All of the fees are actually on the business side, which is why you see businesses that still run on cash only or charge a credit card fee. The credit card network operators, (AMEX, Disc, MC, VISA) are the only option for businesses that want to accept credit cards in the US. You don't see a Debit card fee because it's actually illegal for them to pass along the Debit card processing fee.

So while the average person with the line of credit is happy about this, the businesses are not. In a normal system you would pay for the service being provided. So the person with the card would be responsible for paying to have that access.

Steam does this by making their product (the storefront) free to the average person and charging the developers money to use it. While they also effectively own your games. In a system with plenty of storefronts it might be much more common to see downloadable installation files. That's certainly one way in which they've distorted the market. That used to be very common. It doesn't help that EA, GamePass, and some others who've tried to start storefronts have repeatedly tripped over their own feet. Epic seems to be doing it but they're basically using Steam's business model because there's no other choice as long as Steam exerts it's monopoly power.