How are either of those a monopoly? A monopoly generally means you only have one option and that option is taking advantage of their outrageous market share by jacking up prices.
Where I live the only broadband internet is Comcast which is why I pay 2-3x more for my service than comparable services in areas where they don't have a monopoly (or areas with sane regulations).
Saying that you'll not earn as much money if you don't put your game on steam doesn't mean steam has a monopoly, it just means you're not getting as much reach as you could. Being popular doesn't equate to being a monopoly.