How come people always say to use black spinnerbaits, buzzbaits, and worms at night?

Because they cast a black silhouette against the night sky that is easily visible to fish.

Always BLACK AT NIGHT On www.bassprofessor.com Doug Hannon has explains this on one ofthe videos in the "past episodes" section. I insist you watch all of them videos, and watch how much you learn and improve your fishing  

Have not seen the videos, yet.

In my opinion black is the best color if you want to achieve the best visibility, if you want your bait to clearly stand out. But, do you want to do that? Choosing black only for night fishing would be same as choosing white only for day time bottom presentationsyou will catch only the most aggressive fish in the area.

If you happen to target night hunters (like walleye) during the night, color can be very important. My favorite night crank/stick colors are ghost/blue and ghost/purple, both with slightly reflective sides. Occasionally, as a shock therapy, I will throw glow in dark spoon, spinner or grub. Black at night works for me only if I fish stained water and targeting fish in top 2 feet of water.

Just my 2c