Valve does nothing to enforce their monopoly. They don't stop devs from using other platforms in any way, they even allow devs to generate game keys at no cost and sell them on other websites like Fanatical and Humble.Yup, monopolies are only illegal if they unfairly use their position as a monopoly to keep others from entering the market and challenging them.
Only reason they have a monopoly is because they actually have a good service that attracts customers.