jiyako's sprites (no clever title)

[Vipershark]

I have a legit question...Why is it that pixel artists choose to avoid using FFFFFF white and 000000 black values?

A couple of reasons.

The main one is that pure black and pure white don't play well with MSPaint. Paint doesn't support transparency and turns all transparent parts of images to pure black. If your sprite has pure black in it (on the outline in particular, this will cause the outline to become part of the background) when you go to use the paint bucket to remove the background, any black parts of your sprite will be removed as well. As you can guess, this is really annoying and highly unnecessary when a simple fix is to just use a similar-but-not-pure-black color (and turn off transparency).
White has similar issues and it's a good idea to just not use it.

As far as a technical answer to that question, I can sorta explain it but you probably want to ask someone more art-oriented to explain it better. Basically, pure black is so dark that it "eats" the rest of the colors in your sprite because it sticks out so much. On outlines in particular it's pretty much always better to use a darker version of the color inside the lines. White has similar issues on the opposite end of the spectrum because it's so bright.
It's usually better to use a really desaturated blue for blacks and whites but someone else can probably explain that better than me.