There’s some nuance.

• One of the bigger issues about PC gaming these days is the shader compilation stutter. Valve is able to precompile and upload these for SteamDeck, so a fixed hardware “console” from them will solve this problem too.
• SteamDeck is pretty weak but games are still sort of optimised for it. Same would be true for such a console, making devs focus on a target platform directly.
• Assembling PC yourself can have its own issues. You may not get all the ROPs, or a new Windows update may break your games, or you may occasionally update drivers to make a game more playable without black screens, or wouldn’t turn on with a controller when set up as a “console”. I’m not saying it isn’t doable, but there is an audience for consoles who would love whatever SteamDeck has done for handheld but in console form.