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...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

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.