In principle, it is known that full body harnesses are useful for situations where the center of gravity is higher - e.g. for children, or for Alpine situations involving carrying heavy loads. A full body harness reduces the risk of flipping upside down in case of a fall in such situations in particular.
Nonetheless, I have not seen people wearing a full body harness on the mountains. More pertinently, when I wanted to buy myself a full body harness (for adults), I checked two of the better known manufacturers - Black Diamond and Petzl - the former doesn't even make it, and the latter sells only a single model (and dozens of sit harness models).
Why are full body harnesses uncommon in practical mountaineering?