I highly doubt the diy machine has proper hdmi CEC support.

For a computer that doesn’t matter much, but for the explicit use case of the steam machine (living room gaming) that’s a massive selling point.

By buying into a brand instead of doing diy, you always got: 1. Optimized platform (binary level) 2. Support 3. Ecosystem 4. Ready out of the box 5. Commitment Diy requires 1. Knowing your stuff 2. Handling yourself any compatibility problem 3. Not being taken care of in case of upgrades, upsells, etc

Things you totally have failed to consider, just comparing hardware. It's not about hardware, it's hardware choice FOR A PRODUCT