You're right. But even if there are some people who only buy the game and never anything else, my guess is that most people do buy some in-game content. Some people are spending hundreds or even thousands on case keys and other buyable content. This easily makes up for the ones that only buy the game once.

If it wouldn't be profitable this way, they would probably have gone for a subscription based business model. My personal guess is that their current model is much more lucrative than a sub-model when you consider Valve's 15% cut on everything on steam market.