While Johnson admits that Steam could make money in the short term for hosting ads, in the long term they would lose money. Their main focus is improving the functionality of their software, instead. Advertisements would get in the way of that and ultimately devalue their brand.
In addition, they have new hardware to worry about selling. Valve will want to be on good terms with their customer base now that they’ve released their Steam Controller.
If Johnson’s distaste for advertising is any hint at Valve’s overall plan, then the company’s focus seems to be on making its customers happy and building a loyal base.