Because it is not abusing its position. Monopolies are not an inherent bad, they just tend to start abusing their position, and when they do then they get handled.

This is more common with public companies than private companies though.

Founders have their own life, honor, ethics, desires, etc. which usually help strongly keep the company on a positive track. e.g. Valve Corporation.

This is the American model of anti-trust, very much not the European model (which explicitly targets competition for its own sake even when consumers are not harmed by the monopolistic behavior).

Just because you like a company (and as a consumer, I like Steam) doesn’t mean it’s not acting monopolistically.

Valve is certainly abusing its position. It charges extremely high rents for the services it offers because of its dominance as a marketplace.

It does provide a host of services and does them well, but whether they are value for the platform fee is another question. Using those services creates lock-in and friction to port to other platforms. By providing them as part of the package, Steam has extinguished companies that used to provide those services, meaning that it’s even harder to provide the same functionality elsewhere.

Companies always abuse their position. Its basic capitalism; markets only thrive and are fair when both buyers and sellers have multiple options, and it would be odd to assume this time is the exception.