Because it's another source of power drain that serves "no purpose" or isn't entirely necessary. It's an interesting comparison and I was curious. So I looked.
The typical gaming computer consumes as much power each year as approximately three refrigerators, according to a study from Lawrence Berkeley National Laboratory. The comparison is one of the many eye-opening statistics in a paper written by Nathaniel and Evan Mills on the impact of...
www.forbes.com
According to this article (from 2015) gaming PCs take an estimated 75 TWh per year. As a PC gamer myself, I can tell you that GPUs and CPUs have only increased in power demand in the past 6 years. That figure is definitely higher, now.
The article in the OP states Bitcoin will take 91TWh in 2021.
So, why wouldn't they be mentioned?