They have an overwhelming majority that makes assorted competitors individually irrelevant. Jesus, do I hate having to say “they have an overwhelming majority that makes assorted competitors individually irrelevant,” just because people get in a snit about the word “monopoly.”
You know Standard Oil didn’t own all the oil - right? They peaked around 85% of sales. They had many competitors. Those competitors did not matter.
For every game that’s done well outside Steam, there’s ten that eventually came to Steam and sold massively better than before. That jump is the power Steam wields. That is why we regulate competition, beyond ‘do competitors exist.’