Because then when people go from being people who need things enough that we give them those things to being people who need things slightly less, we stop giving them things. To take a simplified example, if someone needs to be unemployed to qualify for welfare, then while they're unemployed they get support from the state. But let's say they find a job. Now they lose their support from the state. If they job that they get only lets them get the things they need, then they've gone from getting all the things they need and having a bunch of free time, to getting all the things they need and having to work full-time. Who would choose to do that? Giving people who need things the things they need just tells them to not look for employment.

Another problem is how we're supposed to decide what people need. Why not just give people money and let them decide what they need. You'll need some system to handle those who fail to satisfy their basic needs due to problems of mental health (including addiction), but most people are better at deciding what they need for themselves than a bureaucrat is.