Its unknown exactly how much money Valve makes but it is a safe bet they are probably one of the most profitable companies on the planet considering they get a cut of more or less every single PC game sale. Others have larger revenues but, relative to expenditures, they are likely a top 100 if not top 50.

But yeah. Everyone just needs to figure out a billion dollar idea, luck out that people liked them enough to ignore the negatives while everyone else (Stardock, Atari, Gamespy, etc) were getting torn apart, and then maintain an effective monopoly for two decades. Easy.