Again, cheats aside, if people aren't paying taxes it's because the tax-makers set it up that way. Two sorts of decisions come to mind w/ incom tax: Eventually as you go down the income scale, it become's counter-intuitive to tax the poor, with their tiny incomes. Certain amounts of income devoted to necessary and laudable purposes should escape tax: Business expenses, and charitable/medical expenses being the biggest. Somewhere between the two falls what we Canucks call the Basic Personal Exemption that shelters a minimal amount that was intended for, food and shelter, the unavoidable necessities of life.
And even the various Flat Taxes I've heard of have versions of the same.
Of course all these have been elaborated and revised, but the basic principle is the same: Make a decision what's in Taxable Income and what's out, then tax it. 47% of Americans don't make tose decisions. Something in the order of 600 well-paid, job-satisfied legislators looking at fat lobbying contracts and directorships when they can no longer get elected do that.
But whether you need income to pay income tax, and whether you and your tax lawyers defined yours away, or you didn't have any to begin with, or the legislator's decided to ignore yours, the real problem is: Not enough people with enough incomes. Want everyone to pay? Make sure they have the means.