I totally get what you're saying... but to be fair if you did allow people to deduct all the money they spent in a year from their income, it would specifically encourage a lot of people to spend every single dollar they earned just to avoid the taxes.
They'd be like "hang on, if I spend $3000 a month on rent I get taxed on the $1000 I have left over? What if I move to a place that costs $4000 a month? No taxes? That's what I've got to do then!"
Is that sensible? No. The tax they'd pay on the $1000 is way less than the extra $1000 they're planning on spending to avoid it, but people seem to have a real aversion to a little tax, and many would feel they "got something" for the extra money they spent on groceries or restaurants or rent, compared to the taxes which are simply taken from them.
I'm not saying it couldn't work, I'm not saying there aren't better ways to do it, and I'm definitely not saying there aren't weird cliffs in programs like this that lock people into poverty. But I'd be worried that people who are already wrong about graduated tax brackets, for example, would make a lot dumber choices if they felt they could spend $40 on fancier bread to avoid "$40 of taxes"