Are there prebuilt desktop PCs that aren’t? I have personally yet to see one, even though I build my own. Maybe some small form-factor office rigs would be a hassle, but those are not really marketed to usecases where upgrading makes much sense anyway,

That doesn't make sense. Many hardware stores offer an assembly of your hand picked hardware, which gives you 100% control over the components and actually fair prices, as well as the option to use a more sensible case. Of course it costs a bit extra to let them do that and you have to buy everything in one store, which might be more expensive than spreading it out, but it is still better than 90% of those prebuilt systems.
And nothing there is open source, you can install Linux on any computer you want, regardless of where it came from. They just save the Windows license costs.

I'm just calling out those idiotic cases that completely choke your hardware of air. You want an open front (or similar depending on the form factor) to get a bunch of silent fans in to let your system breath properly. Bad airflow will just cause your temps to rise, which also severely increases the noise.