Even AWS tucks mac minis into a rack https://aws.amazon.com/blogs/aws/new-amazon-ec2-m2-pro-mac-i...

Perhaps a policy holdover from the late 90s https://www.wired.com/1997/09/apple-kills-clone-market/

If you take the innards out of any model Mac and 3D print your own mounts, you can get some crazy density. There's basically nothing in them.