Because it's a console, not a portable PC.
The switch offers a console experience. Everything just works and works well.
The deck offers a console-like experience. The majority of PC games work, some may have issues, some may not be suited to the form factor. You can play console games on it but not out of the box.
I say this as someone who doesn't own a switch and uses their deck every night. I absolutely see the type of person who would buy a switch and the type of person who would buy a deck. They both have valid points for doing so and I'd never recommend the other device to them.
When it comes to playing Hades, Balatro or Brotato, I have had zero issues with the deck. It is literally a console experience there. Verified (green) games will just work and are indictive of a console experience. Playable games (yellow) dont represent a console experience. Small text, having to bring up a keyboard manually, launchers.. these things arent something you'd see on a console. Unverified games and emulation require the most tinkering and thats when you really get to experience it as a PC.
In its default state, playing only verified games, only in handheld mode, without external controllers - the deck is a fine machine and offers a console experience. Dock it to a TV, start using more controllers, fiddle with yellow games and that experience is gone. I absolutely appreciate I have the option to do so and not be locked out of it - thats why im a Deck person and not a Switch one.
My point is the deck cant replace a switch and the switch cant replace a deck. They complement each other fine.