I’ve certainly done this with SAS Harnesses coming out the side of my tower case, but you can’t expect a company to make a product with this use case in mind. There exists SFF NAS boxes with room for many drives for this purpose.
I live in ZFS. These box makers often assume you want to run their Raid card, and have optioned up 'to make it better' without understanding ZFS exists, uses its memory efficiently (ignore the 'not on less than 4GB' thing its highly specific to some time in history and de-dupe) and wants a dull-as-ditchwater HBA to talk to the drives. Or, worse yet, they embed the CPU and you buy their packaged solution with their suite of apps and their model of GUI. Like the NAS I moved off, which had a sparc CPU and ceased being able to run, aside from which it was locked to <2TB drives.
You're not wrong btw. I am just saying "no, the home NAS space isn't as easy as you say, if you want to run ZFS yourself" unless I have missed a key term to search for, getting standalone drive enclosures.
I can believe I am not the market. Mainly because very few SBCs have more than one SATA slot, even less more than 2 and many of the M.2 keyed sata cards are listed in a homebrew filestore page with "odd chip, drivers uncommon" -If I was the market I am sure by now, more of them would be mainstream and more web pages would say "sure: its easy!"