Best I can do is a 3.5'' inch SATA to USB adapter case with one of these tiny SSDs glued in

Aren't a lot of the 2.5" ones already empty space?

How big, and how expensive, would a 3.5" SSD be, if it actually filled enough of the space with NAND chips for the form factor to be warranted?

I know right. Why is this not a thing already? I mean I understand the various U.2, U.3, and EDSFF are great for high density data center installs. We have a 1U box in production that could be as high as 1 PB given current densities with E1.L drives but that’s enterprise level stuff. I just want a huge 3.5 SSD I could put in these pro-consumer level NAS boxes or maybe even one I could build myself for my home lab.

Yeah, why aren't there any?

Well, 3.5" SSDs are certainly possible, but 2.5" (or in fact m.2) might just be a better form factor for SSDs. The thing is, an SSD is just a bunch of chips on a PCB, so they really don't need the extra height afforded to them by a 3.5" bay.

You could probably fit 2 pcbs one on top of the other within a 3.5" drive, but that would probably need a third PCB to connect the two which would be more complicated to manufacture and be worse for cooling than using two individual 3.5" or m.2 cards.

Also, for a bunch of reasons smaller is usually better. Generally, it tends to be cheaper to use a few large capacity chips on a small board than it is to use a lot of lower capacity chips on a larger board. Of course fewer parts also means fewer potential points of failure, so better for quality control. And again, smaller cards are better for case airflow and cooling.