Yea but how would someone who isn’t into PC hardware know what a decent GPU, nevermind a decent second hand PC would be, without going to forums, watching reviews, etc.
The whole appeal of the steam machine is it being a console-like PC for people who don’t have the knowledge and/or patience to build their own.
I wouldn’t buy it because I enjoy building myself a bang-for-the-buck PC from used parts and ain’t scared of installing Linux myself. But if you want the benefits of a PC (game availability and prices, flexibility, compatibility) with the benefits of a console (plug-n-play, couch game friendly), a steam machine is pretty much it.
That’s definitely a worse experience in any way besides price.
The steam machine promises a certain point of build quality, support and ease of setup your proposed solution will not.
It’s not the best bang for the buck, sure. But it’s not targeting people who have the know how or patience to either build and/or configure their own system.