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.