Glad someone noticed. If everyone is getting $1000/month, that becomes the new $0/month. Basic necessities, especially rent, will jump in cost; a low-end landlord will know everyone there has another $1000/month to spend, as does everyone looking for an apartment, so rent will jump because if one renter doesn't want to pay more there will be someone else who can. Of course economic complexities will affect this, but that's the short version.
"Supply and demand" is an absolute economic law. Anyone trying to distort that (say, redistributing taxes so all get $1000/month) finds out the hard way. If S&D is not directly addressed in your pet socioeconomic proposal, implementation will fail.
You won't have $1000/mo for food when your landlord raises rent by $1000/mo.
As I said, the actual economics around this are more complicated, but the paraphrase/summary is that if nobody has to work for $1000/mo and everybody gets $1000/mo then $1000/mo will be worth practically nothing, with staples & necessities rising to consume that.
If we exclude rent from consideration, and apply the economics of UBI to just food: the average price of 1 Calorie (of which you need ~2000/day) rises $0.016, or $32/day. For a baseline reference, I regularly make healthy meals at $1/plate. Congrats, you've just increased the cost of food to at least $35 per day - precisely because "everyone now has $1000/month, free".
If we then roughly combine that with housing (as primary costs): I figure a normal baseline poverty minimum of $10/day for living space & utilities plus $3/day food, but then you're adding $32/day available which those necessities will instantly absorb (supply-and-demand) ... ergo you've just increased poverty-level living costs by 3.5x!
Unintended consequence: giving everyone $1000/month increases the cost of a $1 hamburger to $3.50. Now the beggar with $0 has to find/panhandle close to four times as much to afford something barely considered a complete meal. This is not what you had in mind.
"Supply and demand" is an absolute economic law. Anyone trying to distort that (say, redistributing taxes so all get $1000/month) finds out the hard way. If S&D is not directly addressed in your pet socioeconomic proposal, implementation will fail.