First, it makes an enormous difference that Rep. Ryan lives in DC, a city with decent public transportation. To experience the true joy of food stamps, one would need to go to, say, Phoenix. As one of the few people on earth ever to have actually tried to get around Phoenix on public transportation, it's pretty close to impossible: the number of lines is truly pathetic, they don't go to very many places, Phoenix blocks are enormous and walking to the (generally distant) bus station is a bitch, when I was there most of the busses ran around once an hour, making the experience of missing the bus incredibly frustrating, and to top it all off for most of the year taking the bus means walking and waiting in horrible, horrible heat.
Second, consider shopping by bus with children. Small children. The time I remember most clearly, I was shopping with someone who had four small kids. I honestly could not imagine how she managed on her own. (She had just left an abusive relationship, so help from the kids' father was not an option.) How do you carry the bags and hold your kids' hands? What do you do if one of them gets mischievous and runs off? Honestly: I have no clue what the answers to these questions were. I do remember thinking: Good Lord, she has to do this every time she wants to buy groceries?
This makes the absence of grocery stores in poor neighborhoods a much bigger deal. It's not just that heading out to the suburbs isn't an option (let alone taking advantage of the bulk purchasing possibilities of Sam's Club or CostCo, which absolutely require a car); it's that even a trip to the nearest grocery store can turn into something like the Odyssey. If the nearest grocery store requires a bus trip in a city with a lousy bus system and the nearest convenience store does not, it's not surprising that a mother with kids would opt for the latter sometimes. But if $21 is hard to live on when you shop at Safeway, it's a lot harder when you shop at 7/11.
If I was forced to live on $21 of food a week, it'd be really hard. But I'd have the advantage of being in walking distance of Econofoods, which would help immensely. Many people don't have that.
Hilzoy also remarks on a story about how the good Congressman lost his peanut butter and jelly--"4 or maybe 5 meals"--while going through TSA security, thus leaving him 33 cents and a bag of cornmeal for two days. Of course, most poor people aren't flying. But of course, as Hilzoy relates, "there are lots of ways for a jar of peanut butter to come to grief."
Your kid can eat it when you're not looking. Your grocery bag can break, leaving it shattered on the sidewalk. Similarly, the rats and roaches can get into your corn meal, or your wheat bread. There's just no end of catastrophes that might befall your $21 worth of food.
The point isn't to disparage Rep. Ryan's efforts. He's doing a public service here. But it's amazing how, even with the handicaps he placed on himself, he's still a mile away from simulating the actual, lived experience of uncountable poor Americans struggling with the basic problem of feeding themselves and their family.