Because we lose so much energy in general.
Because we don't take care of ourselves.
OP cites Tony Robbins. Anyone who has ever been to any Tony Robbins event knows that he always starts with health (eating right, exercising, and mental attitude) before addressing anything else. Because he already knows what many don't: if you're not feeling right, nothing else matters.
Almost all of us have ample energy in our teens and twenties. But as we get older, we have to make a conscious effort to maintain energy and vitality. Most people I know in their 30s, 40s, and 50s don't do nearly enough. We slow down, put on weight, and lose energy. It happens so gradually that we barely notice. And we blame everything else: family, finances, lifestyle, etc. And we avoid acknowledging the elephant in the room: it's awfully hard to get passionate about anything when we only have enough energy to plop down in front of the TV with a bag of chips.
Most people I know with the energy to accomplish a lot, regardless of age, take care of themselves, especially those working 2 jobs or running a side business.
All of the other reasons (fear of failure, responsibilities, etc.) make sense, but I wonder how many people don't follow their dreams simply because they've run out of energy by not taking good enough care of themselves.
"Sister Marion Irvine did not start running until age 47. She was overweight and smoked two packs a day. Looking for a way to relax after a day as a grade school principal in San Rafael, Sister Marion took up running. ''I needed to use up some pent-up energy, so I decided to give it a go,'' said the 54-year-old Roman Catholic nun, who was soon going 70 miles a week. She started out half-running and half-walking.
In 1984, she qualified for the US Olympic Trials in the marathon by running 2:51 at the age of 54, becoming the oldest to ever qualify for the OTs."
The older you get the easier it is to become overweight, out of shape, etc., so don't lose that energy because when you do it just becomes harder and harder to get it back.