The primary reason is that it's roughly 2.5x the cost. And given that social security is allready the largest expenditure and it's going to be unaffordable in it's current state, I am skeptical.
If that alone is not convincing, consider that most social security recipients receive more than the proposed UBI. And as social security was income tested, you would effectively be transferring social security payments (which are ostensibly a promise/mandatory savings plan) from retirees who paid their dues, to everyone else.
And consider that many benefits are that are covered now would not be available in UBI.
What about people who need intensive nursing? That cannot be covered with their UBI.
I actually like the general idea of replacing a patchwork of county/state/fed programs. I also like the idea of eliminating the welfare cliff.
I just think the amounts being considered are so huge that it's just infeasible.
Of course, you can propose a plan for how this will be paid for.
But sure, I'd accept there'd be medical care it couldn't realistically be expected to cover.