Like how Amazon doesn’t force retail exclusivity, either. They simply dictate prices of other stores.
…One could just not list on Amazon and go out of business. Or list, and have the vast majority of sales go through Amazon since pricing has to be exactly the same anyway. It’s technically a choice, but it’s effectively no choice for many types of products.
Steam is largely the same thing. They are a “soft monopoly” now, they technically don’t enforce exclusivity, but it’s often not a choice for game makers.
Hence basically every indie/AA game I’ve bought recently has been on Steam, in spite of me actively avoiding Steam if I can.