It simply doesn't make sense.
You can slightly-more-than double the capacity by using a 5.25 instead of a 3.5 inch form factor. Doing so can be achieved by putting in one extra platter, too, but at a fraction of the technical problems introduced. Also, consider that in terms of density, during the last decade we're talking about improvement rates which make "double" look like a silly joke.
Nobody really needs double-digit or triple-digit terabyte drives, but those who do need them will use a redundant network of servers with RAID on each server anyway. Still, single-digit terabyte drives (which nobody truly needs either, if you are honest) are readily available in 3.5 inch form factor at neglegible cost.
Further, SSD is evolving much more rapidly than HDD, so you can expect HDD technology to eventually die out anyway. Terabyte SSDs are not precisely a bargain, but they do become kinda affordable. If thought and R&D goes somewhere, it'll be there, nobody will want to spend a lot of BD re-establishing 1960s technology. Go forward 10 years, nobody will be using a spinning, mechanical disk any more. Things need to be small and smaller, and they need to consume less and less energy (because virtually everything is going towards mobile one way or the other).
Among the technical problems with larger platters are not only the ones elaborated very well by Raffzahn and some others which are rather obvious (increased angular momentum in everyday situations, to name one). You also need to consider that in addition to wear being much higher and seek times being much more unreliable than they are already on 3.5 inch, the sequential data rate differs much more in a larger disk.
That, or data density, if you are willing to give away most of the benefit of a larger disk. But then, if you are willing to give this away, why would you use a larger disk in the first place, that doesn't make sense.
So, in addition to having several very real mechanical problems, you also have some practical problems. For example, you need to be capable of writing out data at much higher rates (or spin the drive slower overall) because more area moves under the head in the outer regions. You also need a lot more energy because the head "touches" (well it hopefully doesn't touch!) the same area for a much shorter time.
The larger a disk (assuming the same speed), the more pronounced everything gets. For example, the technical specifications of your drive get a lot more unreliable. It's nice when you can claim a (average) sustained rate of 200 MB/s, but if the actual rate varies from 80 to 300, that's not quite so cool. Same goes for access time. If you write 8ms on the box, and actual times go from 6 to 18 ms, that's not cool PR-wise.
Access time is one of the most dominating factors nowadays, anyway. Throughput is rarely an issue. Anything that makes (or may make) access time worse is bad mojo, even moreso as SSD kinda guarantees a mostly-constant, low access time.