I think people just look at paltry interest rates of say 3%, do a simple calculation, and think that's it, comparing it to current "take home" net wage.
Paying off debt/mortgage/loans/credit cards would immediately reduce people's outgoings, so they'd need less income! Not working would immediately reduce peoples' commuting costs, need to buy work clothes, need to buy expensive coffees/lunches whilst at work, etc., so again, they'd need less income, likewise no childcare costs as you'd be at home to look after your children, again meaning less income needed, and so it goes on.
It's similar to that other thread on here about how little people "need" to live on in retirement, with the same aspects, i.e. no work/commuting costs, no mortgage repayments, etc.
At the end of the day, it's expensive being a worker, especially a middle aged worker with children and a mortgage! Take the work commitment out of it, take the mortgage away, and suddenly your outgoings are a lot lower, hence you don't need anywhere near as much income.