Even if you own a Steam Deck, Nintendo has some attractive value. Nintendo essentially has a monopoly on at least 3 genres of videogame. The entire library of Steam doesn't really have a casual racing game that can go toe-to-toe with Mario Kart. The same can be said for almost any Mario game. Even if a Steam Deck had the games, you'd need 2 decks or an extra controller to get the Switch-style experience. Valve isn't really trying to compete with the Switch on its own turf.