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.