The reason I'm going to buy a Steam Machine instead of building a computer is the verified program.

I've absolutely loved being able to check a store listing page and immediately know if a game will run well on the Steam Deck. Having the same program for a higher end target would be really nice for me.

Also, getting the CEC right is really valuable for me. If I'm building a computer there's no chance I am going to be able to get it to play nice with the TV using just the controller.