Perhaps because it's tricky enough to pick colors that work for a given spritesheet while working from one palette.
That said, there are certain popular color palettes worth considering when you start work, or when doing a recolor anyway. I have my own list here: http://notimetoplay.org/wiki.html?page=color_palettes
I can imagine.
It's also up to the individual artist to decide what he/she is willing to accept. Some platforms have very limited palettes, and will require your graphics to have completely different colours. So it'll be a balance between artistic pride and how much you'd like a sale.
When I create different versions of my tracks, I also provide different qualities in order to achieve different filesizes. That means a 160 kbps 44100 Hz MP3/OGG and a 80 kbps 22050 Hz MP3/OGG, but it also means 2-4 different MOD versions, which I typically label "Low Quality", "Medium Quality", "High Quality" and "Ultra High Quality". Amiga developers often insists on very low filesize, because they want their game to run on a stock Amiga 500, so they'll most likely pick the LQ version (which I'm really not proud of - but I'll leave it up to them).
Various graphics conversion tools will convert a spritesheet in seconds to a different palette. The only question is if the artist can accept the result. :-)
