You're comparing apples to sofas, in my opinion.
The issue with monopolies isn't just their existence, it's their active efforts to keep their monopoly and the negative effects this has on the customer.
Apples monopoly hurts customer, reduces freedom and extracts rent from developers that HAVE NO OTHER CHOICE.
Steams (supposed) monopoly comes entirely from offering the better product to users and developers. Did you know you can generate Steam keys of your game FOR FREE, and sell them away from Steam, giving Steam 0% cut and 99% of the distribution costs?
Compared to Apple, the vast amount of options on PC to acquire software or games means Steam isn't even close to a monopoly. At the very least, they don't have the power to do monopolistic practices and they also just don't abuse them.
Steam is genuinely the closest thing to a perfect company (at least if you like capitalism).