You can buy those though. They may be harder to find in physical retailers, but there are plenty of options out there.
Several years back (2016) I had some EBL NiMH C-size cells that were obviously not AAs in a plastic shell. They were significantly heavier than a AA cell (around the weight of a C-size alkaline) and had a higher MaH rating.
Here's the current product listing: https://www.amazon.com/gp/product/B00FHVM7GC/