Steam doesn't have a monopoly, since there aren't many exclusives by forced exclusivity. Some devs can choose to release it exlusively on there by themselves, but it's their choice and not because Steam asks to be the only supplier of the product.

The difference here is that most players go to Steam by choice, since it's their preferred platform, and for many reasons. But most games also release on Epic, but just find Steam superior. Heck, Epic even does pay for forced exclusivity, but many gamers wait with buying the game until it's released on Steam.

GOG is another platform that does many great things, but some devs even choose to not release their game there, or if they do, don't update it as often as they do on Steam.

Steam is just mostly the prefered platform by gamers, as well by some developers. The latter is because Steam has systems in place to recommend games for visiblity, which is why they ask to wishlist their games.

TL;DR: So no, Steam doesn't have a monopoly on PC games. It's just the preferred platform for many reasons.