Gardening, building things, maintenance, walking to the market etc.
I used to go to the gym but now I have a house and stuff to do it feels insane that I used my muscles to do useless work for so long.
Not that your's isn't valid, some people (like me) have a big surplus of energy that needs to go somewhere and sometimes the best available outlet is lifting weights.
I have a house and stuff, and still go to the gym at 5:30 every morning, as a supplement to my running and cycling.
I guess my cycling is an artificial substitute for riding a horse? ;)