[efitz]

Although the linked page was highly entertaining, I suspect (and this is pure speculation) that the real answer is that, to the people who had to make the original decisions back in the 70s and early 80s, the current design “looked better”.

For subsequent implementations it is likely mostly inertia.

Again this is all speculation but it doesn’t really have to be any more complex than that.

As a side note in the early 80s as a teenager I wanted to write a space game on my Apple //e computer that had a wedge shaped spacecraft like a star destroyer or the spacecraft from Asteroids. I spent many many hours hand drawing the bitmaps for each size and rotation. The shapes always looked a little jagged, except in the +/- 45 degree orientations.