Dr. Infrared 1500W Indoor/Outdoor Heater

Dr. Infrared 1500W Indoor/Outdoor Heater

Why are all (home) heaters rated at 1500W? https://www.youtube.com/watch?v=V-jmSjy2ArM
TLDW: 1500W is the max power a single appliance can draw from a 20A circuit. Because of this the size and shape of the heater makes little difference in how well it heats a space.