

If I don’t like what Comcast charges I don’t do a class action lawsuit.
That’s a poor example, because in many markets, Comcast (or another cable provider) is the only option, or there’s only one other option with much lower top-end speeds (e.g. DSL). So a class-action against Comcast may be a reasonable idea, since they’re an actual monopoly in many markets.
The games industry is different. Steam does have a commanding share of the market, but there’s no real lock-in there, a developer can choose to not publish there and succeed. Minecraft, famously, never released on Steam, and it has been wildly successful. Likewise for Blizzard games, like Starcraft and World of Warcraft.
Maybe a better comparison is grocery store chains? Walmart has something like 60% market share in the US, yet I have successfully been able to completely avoid shopping there.
No, it is where it is because Valve decided it wanted to invest in it outside of it being a launcher/updater for Valve games.
And it’s not really the first. The first was probably Battle.net by Blizzard, which initially was a way to connect players (chat and join games) back in the mid-90s. It wasn’t a game sales/distribution service for many years, but it got there w/ the release of the dedicated desktop app in 2013 and had some of the core features that makes Steam special (chat and match making). In fact, I had the desktop app before I had a Steam account, which I created in ~2013 when Steam came to Linux (I switched to Linux in ~2009, and had played games on Windows for years before that). Blizzard was never interested in becoming a game distribution network, so Battle.net remained largely exclusive to Blizzard titles.
I wouldn’t have bothered w/ Steam if it didn’t provide value. I was fine managing games individually, and I bought many games from Humble Bundle and directly from devs for years before Steam became a thing. I only started preferring Steam when it provided features I couldn’t get elsewhere. These days, it provides so much value since I’m a Linux user, that I honestly don’t consider alternatives, because everything else is painful. Heroic launcher closes that gap substantially, so I’m actually considering buying more from GOG (outside of a handful of old games I can’t find elsewhere).
If another launcher provided better value vs Steam, I’d switch in a heartbeat. I use both Steam and Heroic, and I still prefer Steam because it has great features like controller mapping. But if, say, GOG supported the features I care about on the platform I use, I’d probably switch to GOG because I also care about DRM-free games. But they don’t, so I largely stick to Steam.