There are two types of arguments to make when explaining a topic. You can take something simple and make it complex, or you can take something complex and make it simple and easy to understand.
Doesn't food just go in your body and make muscle or fat? Well yes, but it's much more complicated than that. It's an intricate process involving hormones, digestive enzymes, gut flora, amino acids, fatty acids, and a thousand other things. To add complexity to the problem allows you to understand it's nuance.
I'm partial to the second kind of argument because it often makes information more practical and easier to act on. If someone tells me my diet depends on mTOR receptors, what do I DO with that information? If I'm told to eat vegetables, I just need to decide which ones.
So let's go through a complicated topic and make it much simpler. Why humans should lift weights. Feel free to use this explanation with children, or coworkers who just keep coming back with a face that says 'Why though?' It is in fact, not that complicated.
1. Gravity is a force
2. Your muscles create force to overcome gravity
3. We measure this force in pounds
4. If you can move lots of pounds, you can move yourself and lots of other things
5. If you can't move lots of pounds, you can't move your body or anything else
Option A- Lift weights to increase the force your muscles can produce and lift your great grandchildren in the air, and go for a walk with your spouse before passing away peacefully in your bed that night at the ripe age of 100-and-awersome
Option B- Be confused about what muscles do, ask other people to lift things for you, stay in a chair or bed for most of the day, and invest in a Life Alert for when you fall and can't get up.
Lifting weights isn't just a nice hobby, it is a MUST for anyone living on planet Earth, with gravity and muscles. And we can change those muscles to be healthy and strong and capable. And that quality, the ability to improve, NEVER goes away. You can always get started. You can always pick Option A.