Basically, because flat light is boring!
Disclaimer -- flat light can be just beautiful for female portraits (beauty dish above and reflector below is GREAT for photographing women, where you do not want to see any texture on the skin)
We try to never light paint from the camera. Your camera is usually on Bulb when you light paint, so you have time to walk around and paint from an angle...
In this comparison, taken seconds apart -- you can see FLAT boring light on the top, as compared to side lighting -- creates texture and a 3D look, you can see the rivets, etc. on the bottom, while the top could e a printed paper (lacks texture).
The side lighting creates a more saturated detailed 3D look. You can see so much more detail and every rivet.
#lisatomphotos #lightpainting PhotographybyLisaandTom Tom Cuchara