Why not? Ram is ram, storage is storage.

You can't just buy an ARM or POWER motherboard from one place, a CPU from another place, some RAM sticks from another place, a heatsink/fan, some kind of hard drive (probably NVMe these days), a bunch of cables, and put them all together in your basement/living room and have a working system. With x86, this is pretty normal still. With other architectures, you're going to get a complete, all-in-one system that either 1) has no expandability whatsoever, at least by normal users, or 2) costs a fortune and requires having technicians from the vendor to fly to your location and stay in a hotel for a day or two to do service work on your system for you because you're not allowed to touch it.

But what prevents it from working? I've been building PCs from parts since I was a child.

What about the POWER9-based Talos II systems? Extraordinary niche, I know, but aren't they PC-ish?