I keep seeing "Monopoly" repeated, but I'm having a hard time understanding the logic.

They haven't bought competitors. They don't do anything to hinder others progress in this market, sometime to the detriment of their customers (see: Steam launches another launcher, to launch the game). They haven't openly shown anything anti-competitive, in fact they have stuck to their guns (30% cut) when others have attempted to compete.

What they have done is cultivate the best platform that continues to evolve, add features, and maintain stability. Consumers continue to choose to use Steam overwhelmingly, but outside of Valve's own games, there is no threat of exclusivity or punishment.

It's the opposite of monopolistic behavior. Any company is free to compete, build their own platform, and offer software. It's expensive, and tricky to get right, but nothing is stopping them, Valve included.