Valve have done some nice things. Big Picture and controller support are unrivalled. Steam Workshop is... alright. Easier to use than modding sites, yeah, but they never were hard to use to be honest.I hate this, "they print money they don't need to do anything", but while they haven't made any games they are still, by far, the best online store for PC. And it's not like there isn't competition EA, Microsoft and Ubisoft have been at it for more than a decade and Epic is going into 5 years; yet all of them are years behind compared to Steam, not just on the core functionality but the extra community stuff, like Steam Input and Steam Workshop or even something as simple as the community forums.
Steam doesn't print money by existing, it prints money because it's far above the rest.
Steam still has nasty DRM (sorry, I don't care that it's 'better' than others), looks like it's from the late 2000s, and most of the forums are a cesspit.
No, most of it is them getting lots of money for doing very little and benefiting so greatly from being a first mover. Then add in customer inertia and a few Steam/Valve fanbois* on top, and there you have it.
*Who will often vehmently attack any attempts at any other PC digital games stores trying to get some of the market. Usually with 'I want all my games in one place' as their main reason.