> live on much less than this.

They do not actually live on less, they sacrifice their health or well-being in order to meet the constraint.

I would argue the calculator grossly underestimates necessities because most of these jobs are not doable in old age, so you need to account for saving $1 for each $1 you make, to support yourself while old. You also need an emergency fund, because in the US you get billed $1000 for the most random shit at the most random time.

I got billed $5000 randomly for an echodardiogram because insurance didn't pay for it despite them saying they would. At least I have $5K to spare, but considering that can happen, that needs to be considered a basic necessity.