The point of the beige box is not too look cool and bursting with technological power. Quite the opposite. The beige box is deliberately non-threatening. It needs to look harmless so as not to frighten the vast majority of the intended user audience - people who have to use PCs, but don't really like them.
If you're a geek, go ahead and get your iMac or your SMP Wintel box in a matte black case. But your Mom probably prefers the nice beige system. It's not threatening, it doesn't clash with the decor.
Stereo systems are black because they're supposed to emit coolness. IMacs come in a rainbow to appeal to the artistes, the creative types who are the main market. And PC cases are beige to appeal to your boss, your Mom, and the nice but computer phobic checkout clerks.
"It's a beige alert." - Futurama, Brannigan Begin Again
Mea culpa: In the mid-'90s, the tyranny of beige finally began to crack, and 'powerful' computers began to become available in black plastic boxes. I got all 'alpha geek' and demanded one, which to my surprise I actually got ... but I didn't get to keep it long, because the marketroids decided to take it on their next dog and pony show and I never saw it again. Nowadays, of course, my Thinkpad is black, but it just ain't cool.