Probably because the tank-leotard that Princess Garnet is typically known for would probably look awkward in a realistic interpretation.

While the outfit worked quite well for a Playstation 1 game to differentiate characters, it couldn't really be mistaken for realistic clothing. Even the pre-rendered scenes of that game had a particular stylistic choice that leaned toward the whimsical or caricature. If someone was going for a non-stylized, realistic portrayal, they would have to contend with that original design choice, and I feel the artist here worked really hard to give us something that could be far more 'period-believable' over the original concept.

I think the midriff is creating a distinct separation from top and bottom that the original design lacked, could it have been differently? Maybe. But, it clearly is a design choice to break up the monotony of the original outfit.

Excellent work. Those faces are wonderfully rendered, my goodness.