JSMa, we were over this one before, but the families that make it work on a shoestring tend to do it because both parents agreed, and they have the cost of maintaining one household, not two.
Yes, as a single mom I lived at home for a while. And I worked crazy hours. My mom was my daycare. When my ex eventually got a job he was making ~$26K/year. I could've gotten $300/month from him, had I felt like spending the $ to go back to court. That in itself would not have been enough for me to move out and be on my own. It would have been enough to cover his 1/2 of my DD's food and clothing for sure and probably some other things. Based on that he would have been fulfilling his obligation to contribute to the support of the child born into our marriage. The responsibility to create and support a household for us (I have sole custody) was mine.
My perception, based only on my own experiences, is that many ex spouses expect the former partner to contribute as much to the equation as when they were a couple. And there's often a chip on the shoulder that he doesn't. [Admittedly here I am not talking about women who are struggling just to put food on the table. These are middle (to upper middle) class women residing in a suburban area.]
Of course, you could flip this and say that step-moms want their partner to contribute 100% to their household, they resesnt that their partner can't be counted on for more, etc. But I believe many SMs go into the situation with a more realistic picture. They know they (or the children they choose to have) don't get 100% of the pie. Often they are subject to an ex-spouse who hasn't quite accepted that same reality.