I think it's the idea that there's no coming bacck, no second chances, it's final, and unavoidable. You're gonna inescapably miss that last brownie.
The sense that a person's done everything they desire in life, and have no regrets that ought to be made better is hard one to achieve. There's no coming back to tell your kids how much you love them, or to cure world hunger.
I find deadlines for collegework scary enough.
That's one factor in procrastination, in my eyes, it's never going to be good enough, so why bother starting? There's no big sense of completeness if you know the end is coming, and if you don't, you're not even gonna have a chance to fulfill your life.