It's not just a packed lunch, it's a whole ethos, a whole way of life. Obviously you need a high enough income in the first place to save a million, but lots of people who "could", don't get that kind of savings because they spend their income.
A minimum wage shop worker will never save a million, but a relatively successful professional could well achieve that by saving like made, not spending, and making sensible investments, etc.
It's like people who sneer at the old "pennywise and pound foolish" saying - they sneer that saving the pennies won't achieve anything, but that's not the point, which is that people who are careful with small amounts get into the habit of shopping around, always looking for offers, always looking for value for money, always carefully considering when buying and not being impulsive etc.