What's your reasoning?
From an economic point of view I think it costs too damn much if the monetary amount is set anywhere near high enough to achieve the desired effect (that most often being described as enough to cover bare minimum cost of living). The govt spending that much money will require either enormous cutbacks in spending or a huge spike in deficit/debt, all for something that will contribute next to nothing to the economy. I am not a trained economist so I consider this to be the weaker of my two main objections.
From a moral point of view I don't like the idea of taking taxpayer money and distributing that it in such a way that said taxpayer has no idea how their money if being spent. This is an objection I have to certain other government programs too, not just UBI. I could go on for a long time on why I hold this position but for the sake of a shorter post I will not do so.
You probably agree with these two points but where I assume (possibly incorrectly) that we would disagree based on what few posts I have seen of yours on the forum and the fact that you identify as a conservative on your profile is that you probably (again I might be wrong) subscribe to the belief that implementing a UBI would encourage people capable of working to instead not do so and simply live off their new monthly government check. This is a common objection I have heard to UBI and quite honestly I find this objection even more laughable than the idea of UBI itself given the amounts involved in most UBI proposals and my experience in successfully fighting my way out of poverty over the last decade or so. Again I do not know for sure that you hold this view, maybe you don't, but it does seem like a common one.
And do you have a theory for why so many otherwise smart people think it is viable?
I have not looked closely enough at the arguments in favor of UBI to have an opinion on this. I doubt it is as simple as 'they are all just stupid/lazy' though.