Exercise is costly in time, and, therefore, money.
As a sort of self-performed experiment, I've exercised regularly for the last 2 years, on average of 3.5 times a week (every other day).
This required 1.25 hrs for the workout itself, another .25 hrs for the added commute, $60 for the gym membership per month, an increase in calorie consumption to compensate of about 30%, hence additional food costs of around $100/month.
That's a total of 273 hours in a year in opportunity cost and $1920 in various expenses.
Assuming a semi-skilled wage of $15/hr, that's $4095 + $1920 ~ $6000 opportunity cost per year to exercise and stay healthy.
$6000 is a make or break amount of money for poor people. I can afford it relatively easily, considering how much health benefits I receive from it, but living paycheck to paycheck, most of them likely can't.