Why no 18bit RTG modes for Amigas?

[AmigaMance]

I remember that it had become a standard on PC's and it was called High Color, or something? Neither CGX or P96 support 18bit modes. You go straight from 16 to 24bit modes.
 I think it would have been a good compromise between speed/memory usage and image quality.

[AmigaMance]

I don't. I do however remember that the VGA palette was 18bit.

 I remember reading some pc game reviews, back in the early 90's, where some games were using 18bit screens and the editors called it high color. There is a possibility that i'm totally wrong about it.

How exactly?

To get 18bit you'd still need to have 3 bytes per pixel, or use some kind of weirdo mode where the 6 extra bits would always be used for the next pixel. That'd be prohibilitely complex and slow to work with.

 Ah, ok. I just followed the simplistic reasoning "less colors, more speed".