Honestly it is tempting. I do have an old Haswell-era industrial motherboard that would manage the task just fine, and I've definitely considered this path.

That said, I'd probably spend about as much on a power supply, case, and NIC for that machine as I would on just buying a Unifi gateway, and theirs comes with an integrated UI for the APs. I'm past the stage of life where I find joy in tinkering with the infrastructure I need to do my job (WFH) so I'll probably still just go off the shelf.