> but spending 5 hours a week at the gym isn't a big time investment.

That's a sizeable investment when you have kids. Which is fine -- but you definitely have to stop doing something else in order to do it. If like me (and perhaps parent comment) you _already_ don't passively consume media, go to bed early, etc, you really do have to trade something for it. And (again, perhaps like OP) if you are already healthy and fit (ex: I can play soccer for 2 hours) there's even less benefit (relative to your other pursuits). Which is all actually to say -- there are many people for whom discipline is certainly _not_ the issue.