Those motherboards have nothing to do with ATX. The rackmountable "blades" are 19" wide and ~24" long. They use redundant custom "jet engine" PSU with custom connectors. Even their PCI-E isn't standard. They are custom built for each generation of servers and mass produced.
Even higher end workstations like the various HP "Z" series and Proliant are custom.
(disclaimer: I have my (large, ~120 Units) homelab/self_employed_datacenter on ATX Rackmount 4U cases with watercooling. I am used to buying servers for the CPU and RAM as others do in this thread. I use consumer gears too, but they die young under full C++/C embedded CI jobs 24/7. I use 4U cases because of the unbearable noise of the blades)
I am in Quebec, power is cheap to the point of this being irrelevant. Heating is also required a large part of the year (including until yesterday because the weather has been horrible so far this "summer"...). Plus, well, tax credit for business expenses make that "less than free".
> and how loud your racks are?
Watercooling and passive PSUs makes it silent. I hate noise.
> Can I ask what you're doing with that home lab
Beside all the usual services for a small business (phone, email, storage, backup, routing, etc), it is mostly an oVirt (RedHat Enterprise Virtualization) private cloud running Docker VMs used by the CI to compile C/C++, build embedded device firmware images and run tests. The extra horse power (and least power efficient) nodes are woken up use wake-up-on-lan and boot a template using PXE from the GlusterFS distributed NAS. They are shut down when the load goes down. Only the big 7U case with the double1 140mm watercooling fans is open all the time. It is the "head" of the cloud cluster (but in theory is configured to auto migrate in case of hardware failure to the dual 120mm 4U case, but honestly I never tried the automatic migration).
Not very pretty, but good enough. https://imgur.com/a/hnlInSz (yes, there is some overlap between those 2 pictures because I moved some units between taking them, some units have been traded for others too).
Like Elv13 already said: the form factor is totally different, so it won't work in any desktop enclosure or with a standard power supply.
But apart from that, server boards sometimes don't have a storage controller, no audio, only a few USB2 port. Most have onboard VGA graphics that you can't turn off, or the BIOS won't support outputting video with a different graphics card.
And then there are usually few Windows drivers and you could get into heaps of issues with (the lack of) UEFI if you want to run a desktop version of Windows.
Server boards are custom build for datacenter application and just aren't suitable for workstation use.
