I know one reason why we either order shirts as a group or get extremely specific is that some people just don't HAVE fashion sense.
For example: "Just wear pink T-shirts" will result in everything from baby pink to watermelon to fuschia to colors that aren't really PINK to anyone else but the wearer.
If you don't specify black belts, you might end up with a girl wearing a white belt... or worse no belt at all, so when she bends down during the song, you see her butt crack
If you say just wear black bottoms and black shoes, you're going to end up with girls wearing capris and black clogs... and y'all know that's not cute!
I know all of these examples are slightly exaggerated. And you COULD just say, "Pink shirts, black bottoms, black shoes" and then veto anything that falls under the line of acceptable. BUT... it's so much EASIER and SIMPLER to be more specific so everyone knows what works and what doesn't.