30% of someone making minimum wage isn't the same thing as 30% of someone making 500k/year.
I'm fairly well off. If housing took 70% of my after-tax income, I'd still be able to live reasonably well (and would have one hell of a badass home to go with it).
My food, my phone, my electricity bill, my insurance, my transportation, they cost me the same as they cost someone else living in the same city making 1/4th of the money. Since real estate is wealth storage at worse and an investment at best, I can pour everything else I make it into it. It's not particularly optimal, but I -can- do it if I so choose.
The end result is that money available for housing goes up in a curve as you go up the income bracket. People below the threshold get screwed.