I think it's really just because there are no objective and enforced standards for it. I used to have a kitchen with a pretty high ceiling and a fume extractor mounted way up high, so high it was controlled by an IR remote, and it worked great while making little sound. Currently I have one of those integrated with a microwave oven that makes a lot of noise, blocks my view of half the range, and doesn't work at all. Plus even if it did work, it would vent right into my eyes, not to the outdoors.

That sounds like horrible usability; it's going to be in the way all the time, especially if you're a tad tall. I bet that's the far more important reason than aesthetics. I already don't like the regular ones because I need to be careful not to bump my head in to it.