Because a third party publisher is a supplemental actor to a console market when they make games for both platforms. If the one getting its ass kicked makes those games more scarce on its competitor's console, it becomes more difficult for you, the consumer, to choose one, which means the market got more competitive. Or you buy both, which means both competitors (Nintendo doesn't really count here) are healthy for each getting your sale. If your default answer was to buy a PlayStation and to hell with Xbox, that's less competitive.
The third party video game market is in no danger of monopolizing, on the other hand. Ubisoft/EA/ActiBlizz/Take Two all put their eggs in fewer and fewer baskets, and now the Devolver Digitals, Anna Purnas, TinyBuilds, Focus Homes, Paradoxes, and Embracers of the world are growing to fill the market voids those big publishers left by putting out fewer games.