This is a really good question. On the one hand an argument can be made that they’re that way because the triangle is the original, most authentic shape for a scone. That’s true, since the original Scottish scones were fairly large, flat cakes, cooked on griddle, then sliced into sections like a pizza. Yet there’s something disingenuous about claiming the mantle of authenticity when you make scones that look like this.
Me, I think it’s all about marketing. If American scones were small and round instead of giant and triangular, people would just say “Hey! That’s a really lousy biscuit!” And that would be that. As it is, people make allowances for their density and pressboard dryness because they assume they’re supposed to be that way.