How do you incentivize people to work? Pay them more.
If mass unemployment was a substantial problem, this may well be an acceptable tradeoff, but in the current economy it is not.
If you get just enough to survive from the government, and employers try to reduce wages because you'll net out the same, you probably won't accept the job. It's not worth your time. Even as a dirt poor person, your time is valuable. An employer needs to pay you more so that it's worth your effort again. You have much higher freedom to shop around too.
If the government does not pay you enough to survive (non basic income), you're still in a precarious position, but you are still in a BETTER position than you were without the funds. This will RAISE wages, which will in turn RAISE prices, until the equilibrium is found where the UBI is distinctly not sufficient and you need to work to survive. You will still be poor. You will make more wages, but have about the same level of real wealth. You will be a bit safer due the guaranteed portion of income.
Until globalization kicks in and makes your specific local circumstances much worse.
A UBI also opens up the possibility of removing the minimum wage which not only allows for more people to obtain jobs, but also raises the competitiveness with other countries, potentially (it depends on whether the minimum wage is actually effective in raising wages above the market rate).