While you can do that with a Steam Deck or Nintendo Switch, that was not their primary design. Its no different than plugging in a laptop to a TV with a wireless controller and gaming on that. That is not a console, it was not designed as such.

Additionally, the quality of gaming experience you get from a Switch is worse than on Steam Deck for intensive games, even for native Switch games sometimes. But the quality of games on the Steam Deck is worse than a purpose built game console like the Xbox Series X or PS5. Keep in mind the Steam Deck is 2 years newer than the Xbox Series X or PS5. While the experience may be comparable on some less intensive games, the console experience is objectively better.

Steam Deck and Nintendo Switch are not competition to Xbox or PlayStation. Valve and Nintendo don't view them as such, and neither do consumers.