Testosterone levels drop and the body slows down. These aging symptoms reduce the muscle tone and drive to exercise. Fat collects on the slacker stomach muscles and you have belly bloat. If you aren't seriously into how you look, the effort it takes to keep looking great isn't worth forsaking your other pursuits in life.
My own pet theory is that it's down to how the body handles trans fats.
Either that or too much beer!
You don't exercise and tend to drink more, of anything.. You're too tired to exercise mostly anyway. Life just wears you down. Like a car. Great when brand ne (usually) but as you use it then it starts to wear, bits stop working, need replacement, just fall off and eventually it gives up. So do we.