You can't live off of only bananas. Not only that, but it's about one bunch of bananas for ~$2 in season where I was in California. Dunno what they're like here in Ohio. So that's not a lot of food for 2 dollars.

Let's also take into account actually getting those bananas. It's easy to get fast food. It's fast, you can eat it there or walking home or on the bus. You get hot food near instantaneously and can prevent yourself from starving. McDonald's is all over the place, you can usually have one within easy walking distance of work or home. They are often near bus stops.

Grocery stores aren't as easy to get to a lot of the time. And even if they are, you can't necessarily afford the time to getting to the grocery store. Especially since you're going there for what, one thing, maybe you can afford two?

Time is as significant a factor as straight up money cost when you have no easy transportation and can't afford more than $2-4 at a time for any given day.