Not only do you have to build them, but in all those builds the cases are several times larger than the Steam Machine, so they are not actually equivalent. If you care about the form factor, there is nothing directly comparable.

These options are up to 10 times bigger than the SteamMachine, approach almost twice the power draw, and are up to 2.8 times louder than the SteamMachine. They aren't even close to comparable... so why exactly are they making this apples to oranges comparison? No one is assembling a PC to put under their TV, and even if they are, ignoring the obvious form factor, heat and loudness issues... they'll most likely still want to install SteamOS for a "console-like" experience, which is still a win for Valve...

First option:

* 35 litre case (~10 times bigger than the SteamMachine) * ~250W power draw (vs. ~185W SteamMachine) * 32-36 dBA (vs. ~23 dBA SteamMachine)

Second option:

* 35 litre case (~10 times bigger than the SteamMachine) * ~285W power draw (vs. ~185W SteamMachine) * 32-36 dBA (vs. ~23 dBA SteamMachine)

Third option:

* 19.2 litre case (~5.5 times bigger than the SteamMachine) * ~295W power draw (vs. ~185W SteamMachine) * 34-38 dBA (vs. ~23 dBA SteamMachine)

Fourth option:

* 19.2 litre case (~5.5 times bigger than the SteamMachine) * ~275W power draw (vs. ~185W SteamMachine) * 32-35 dBA (vs. ~23 dBA SteamMachine)

This is the old “why buy a prebuilt PC when you can DIY for less”. Which is true, but completely misses the point that may people will happily pay to 1) not have to build it, and 2) have e2e warranty, and 3) have someone else do the software setup.

We can debate if it’s the most efficient use of money for a technical person, but it’s indisputable that many people get enough value to pay for the prebuilt.