They don't own the OS, they don't (until very recently) own the hardware, they haven't really made any major uncompetitive or anti-consumer moves I'm aware of, and they provide a service that the majority of devs consider worth it.
I guess you could argue they're taking advantage of a bit of a "natural monopoly", but there's still plenty of room for other people to eat their lunch, and things like itch seem to have carved out a niche for devs that would rather keep their money than get the additional services Steam offers.
I don't think Steam is flawless, but for how powerful they are, they sure seem a lot less evil than almost every other large corporation.
Valve built a platform that gamers like, and gamers like it for all the choices Valve made.
I also find it interesting you chose "not allowing reselling" as a thing that would have made users not like steam... but not allowing reselling is probably the feature that game developers like the most! I wouldn't be surprised if developers would choose to keep the 30% fee over dropping the fee but changing to allow reselling.