They are a monopoly because they've had the best product on the market consistently for 15 years. There used to be huge resistance to them and their drm from gamers, but they have shown over many years that they are trustworthy, unlike others that have tried this.
This is not an Apple or Google store situation where proper competition could not exist. They were always up against giants like Microsoft, EA, Ubisoft or more recently Epic.
Today, yes, I agree. It's really hard to compete with them anymore. But 15 years ago when everyone was rushing to capture the market, there were many opportunities to do so. Steam and valve were never infallible, but at least they took feedback and stayed consistent, unlike their competitors.