I base it on using pcpartpicker to look for modern motherboards (last 2-3 chipset generations) with more than 6 sata ports or more than 3 nvme m.2 slots . Results are few and far between, and never together. And if they are when you look at the motherboard errata they often disable 1 of the m.2 nvme when the second GPU slot is populated, or an entire handful of the sata ports.
You can just buy an HP Z Workstation and you'll never fill up all those SSD slots. Plus SATA HBAs are $10/port so this seems like a weird hill upon which to die.
HP workstations are pretty locked down compared to a build it yourself desktop. Like, if some particular type of fan is not connected it won't boot, no way to disable it in BIOS/UEFI settings.
re: HBAs in general, yeah, I'll admit it's weird but I really don't like having to use external active controller cards for reading my storage. I just want my motherboard to do it to keep things simple.
On what do you base this? The only thing I can think of is that a lot of unpopulated x1 PCIe slots got replaced with the much more useful m.2 slot.