Steam is the most dominant game tool on the planet and landed when there was not yet a market for it. Very few other projects will get to the level of success it has in any sector, anywhere.
GabeN was also a MS developer back in the day and likely would have been well off regardless, but he didn't need to play the YC A-B-let's-shoehorn-AI bullshit games that are 100% de rigeour for all startups in 2025.
From what I understand, Gabe/Valve almost went bust during Half Life's development. His gamble paid off when that turned into a runaway success, but he still could have lost it when he bet again on HL2 and Steam; at the time it was extremely controversial to make those a package deal. If Half Life 2 had been not quite as good as it turned out to be, it could have turned out to be a studio with a one hit wonder that burned their goodwill with some sketchy DRM sort of scheme on their second game.
GabeN was also a MS developer back in the day and likely would have been well off regardless, but he didn't need to play the YC A-B-let's-shoehorn-AI bullshit games that are 100% de rigeour for all startups in 2025.