It's true that most people do manage to consume a basket of goods costing at least as much as reasonable basic income levels. Even the homeless end up consuming many tens of thousands of dollars every year, in costs of health care and social services. (More, in fact, than the simply impoverished do.)

But that suggests a key point: the basket of goods and time that people consume while below the poverty line is not necessarily what they would choose to consume if they simply received some amount of money equivalent to what one could reasonably expect to survive on. For instance, a huge chunk of the money we ostensibly give to people goes to Medicaid, which effectively redirects aid to an expensive program that doesn't even seem to improve health outcomes. What the money would flow into instead would cause inflation in those goods, but at the same time capital would flow to the production and provision of those same goods as more people were buying more of them. The net result is increased consumption of the replacement goods coupled with an increase in price somewhere between 0% and the percentage increase of the demand going for them. The exact value depends on elasticities and how many new ways capital can find to supply those goods.

There are welfare programs that do pass out real money, but they force a different kind of consumption: of time. Instead of consuming hours as an individual knows best, we ask them to send out dozens of ineffective applications. I don't know if they even have good choices: maybe these people are just screwed. But I do know that they probably know a lot better about how to get out of their situation--within the constraints of their particular levels of knowledge and self-control--than I do.

There are also psychological aspects that are pretty important. Living in a constant state of staving off being kicked onto the street forces a focus on short-term thinking, at the expense of medium- and long-term thinking. This is more behavioral than micro economics, but it's as real a factor as any other, and definitely changes what and how people choose to consume.

It's worth noting that I've left off all discussion of people above the basic income level (which, to be clear, I'm imagining as being around $10k), who constitute a significant majority of Americans. Two points I'd like to mention:

1) A lot of people fall into a benefits trap, where if they work they'll be punished. Literally they face a greater than 100% tax rate. For people on disability, this can actually be from 1,000% to 10,000%. This is horrific from an economic point of view, and horrific from the perspective of human dignity and autonomy. A basic income is one way of totally removing those obstacles to participating in the labor force.

2) People at $40k and above theoretically shouldn't be affected much by my suggested basic income, aside from a real but manageable tax hike. In practice, though, I suspect the behavioral aspect would be significant, if they know that they can quit their jobs without having to worry about bare minimum needs (though this is clearly just a strong suspicion on my part--I have difficulty imagining a good test for it). Instead of worrying about destroying their savings, they can go out and try new, risky ideas. I think we live in an era where there's a great need for individual entrepreneurship in both monetizable and non-monetizable goods, so that's something I embrace.