The price that shows up on no "DIY vs. Steam Machine" comparison is the integrator tax. You literally cannot build a PC this compact, this well integrated, this silent and with all the custom HDMI trickery yourself with parts available from NewEgg. You just *can't* unless you are willing to spend substantial amounts of time and actually have the skills required to pull it off.
But I also get the the Steam Machine sits in a really weird spot the Steam Deck didn't launch into:
The Steam Deck was and largely still is competing with the Nintendo Switch. And here it offered a really compelling value: vast back catalogue of games, a lot of which you probably already own, now on the go in basically a hand-held PC. Awesome.
The value proposition of the Steam Machine is: small neat silent box that also has HDMI-CEC. Those are two very specific features to care about, and I feel for most people that already have games on Steam they lose to "most fps per dollar". Now for people that are *new* to steam... it's a really tough sell compared to the PS5 or even the XBox.
The bottom line is this: people want a console with console features at console prices that also is a PC that does PC things with PC performance. That simply doesn't square. The current hardware economics just make this even worse.