It's not a de jure monopoly, it's a de facto monopoly. When a gamer thinks "I wanna buy this game", 90% of them go immediately to Steam.
It's a monopoly in the sense that it's practically financial suicide for a games developer to NOT sell their game through Steam. Whether or not they really "have to" is besides the point if the reality is they'll not make a positive return on their game if they skip out on selling on Steam.
A few big publishers like EA avoid this by creating their own alternative stores and just hoping gamers will deal with it and install Origin, but most games studios have no choice but to accept that they're going to lose 30% because the reality is gamers expect to be able to buy any game on Steam.
It's a monopoly in the sense that it's practically financial suicide for a games developer to NOT sell their game through Steam. Whether or not they really "have to" is besides the point if the reality is they'll not make a positive return on their game if they skip out on selling on Steam.
A few big publishers like EA avoid this by creating their own alternative stores and just hoping gamers will deal with it and install Origin, but most games studios have no choice but to accept that they're going to lose 30% because the reality is gamers expect to be able to buy any game on Steam.