Valve, with whopping >70% share is a monopoly.
I don't know anything about Valve, but monopoly is where the big money is. Happens all the time in software and the internet where it's easy to do if you are first to the game and make monopoly your goal. Even a shitty company can easily parlay an early dominance into eternal dominance and big $$$. Microsoft is a great example. Ebay. Facebook.
Remember when there were a bunch of different online auction sites and how cheap it was? Ebay was the worst one but they were first and always the biggest. Briefly there was a site where you could go to browse listings from all the auction sites at once. Great for consumers and competition. Ebay sicced their lawyers on it and got it shut down. Competition disappeared and fees went up by ~4x.
Anyway, this is way OT for this thread and has nothing to do with video cards....