A monopoly (from Greek μόνος, mónos, 'single, alone' and πωλεῖν, pōleîn, 'to sell') is a market in which one person or company is the only supplier of a particular good or service. A monopoly is characterized by a lack of economic competition to produce a particular thing, a lack of viable substitute goods, and the possibility of a high monopoly price well above the seller's marginal cost that leads to a high monopoly profit.[1]

Steam is not the only supplier of particular goods, they do not own the market, they have not the highest price and do not lack competition. It is just that their service is far better than whatever competition offers. Nothing stops Microsoft, EA and Epic to implement same features Steam does. Like, literally nothing. These companies have money to do so. They just chose not to.