I think an argument is that because many people live paycheck to paycheck, investing simply isn't an option. Many costs in life are somewhat fixed

I buy similar groceries compared to someone who makes half what I make, and compared to someone who makes twice what I make; as a fraction of income it's a huge spread.

What this means is that making twice as much money doesn't mean you get to invest twice as much

you can invest way more, because the difference in income is largely disposable.