It’s not TOO hard to build a 5- or 8- drive mini-ITX system, and I like my Jonsbo case well enough.
Main issues:
mini-ITX server motherboards are expensive (even asrock-rack).
mini-ITX server motherboards use OCuLink to connect hard drives. The cables aren’t sold to consumers, and there are two wiring standards for OCuLink-SATA cables that are unmarked (I got lucky on ebay).
mini-ITX server motherboards use unbuffered ECC ram in a laptop form factor, which is very specific and again expensive.
It’s not TOO hard to build a 5- or 8- drive mini-ITX system, and I like my Jonsbo case well enough.
Main issues:
So the technical barriers are real.