Because in the long term you do plan to replace existing welfare systems.
Think of UBI as another lever in a countries' monetary and fiscal policies. In fact is it completely insane that this is not already in the mix from a purely pragmatic perspective.
It's an authortarian dream. Got a fine? Just deduct it. Putting people on the take gives government significantly more leverage over them. Just wait till China pairs it with their social credit score.
it...is? what else would you call sending stimulus checks to nearly every American? however shortlived, we effectively had UBI, helicopter money, or a negative income tax, whatever you wish to call it