The PCs they built don't even work as well anyway.
They're typically
- more expensive
- larger
- require careful research to get parts that fit
- require a few hours of assembly
- can't be powered on/off along with the other HDMI devices in your home theatre setup via HDMI-CEC
And run hot and loud anyway.
Sure if your only objective is "max fps per dollar" then other "better" alternatives exist.
But vs. say, a ps5 - the steam machine is a linux box. you can run whatever on it. You can run retro games on it. You can do work on it.
And vs. other small form factor linux builds - its only worse if you consider your time free, the size advantage meaningless and the HDMI-CEC useless.
Most people want those things and the few dollars you'd save building your own aren't worth what you give up outside of the very specific max fps per dollar niche.