As I said, I do have a shelf for computers. Most of my computers sit in this shelf — several of those which do have a case, and all of those which don't have a case. There is also power distribution and monitoring, ethernet switch, and a KVM switch placed in this shelf. It's about as space efficient as 19" racks but probably cheaper and doesn't limit me to 4U coolers for CPUs and GPUs.
But here is the big caveat: The shelf is located where I can power it from two 230V 16A circuits. Or even from three of such circuits if I put a cable from the next room. That's how I can have rather many computers in a single room, and thus it's worthwhile to cram them into a rack or shelf. (And the rack or shelf makes computer cases superfluous.)