It's worth drawing the distinction that Steam, as opposed to Apple and their app store, does not hold an exclusive monopoly and cannot dictate where users can install software from. If a Dev doesn't like Steam, there are other publishers and store fronts that they can peddle their wares through. Similarly users can go elsewhere to buy and install, even direct from the manufacturer.

Steam being the de facto choice is another issue entirely, and yet another discussion for their fee structure.