Or you can buy one off the shelf from, say, Puget Systems.

They're not vertically integrated, they're sourcing and assembling the same parts you and I could get (and some we can't: they have graphics cards in stock!)

That's for folks that don't need OSX, that like build quality in the form of sturdy, repairable, maintainable tools rather than glossy but glued-together disposable ones, and want Ryzen or Intel performance that will beat a Mac Studio without the hassle.