Well, that’s certainly possible, but 2.5" 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 also be more complicated to manufacture and 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.