Though skill acquisition principles show that so called functional training is not based on sound science, perhaps the greater confidence it instills, plus the sense of accomplishment that comes from getting better at any skill can be very habit forming (addictive?).  This may explain the popularity no matter how bizarre of disconnected from anything you do as a daily activity it is.  

Confidence, however can be a very addictive drug, and cause people to continue to seek it, even in the face of getting injuries.  (not unlike an actual drug that you know is killing you, but the "high" is just too good)

Functional training soon starts to mimic sports, as in Crossfit games, and in fact, I suggest that the "cultural norm" of being obsessed with sports, and especially in pushing our kids to participate has a very dark side as exemplified by the injury statistics.

https://www.hopkinsmedicine.org/health/conditions-and-diseases/sports-injuries/sports-injury-statistics