Several reasons. Firstly, people don't simply move to the cheapest tax location, as being in America gives you access to things like...well, I don't know why people like being in America, but many of you plainly do. Secondly, because there's no real evidence that a basic income would cause people either to stop working or to choose not to work. Thirdly, characterising taxation as theft is sophistry. Clearly some uses of tax are more worthy than others; that doesn't make the principle unjust of itself. Fourthly, we already tax rich people and give it to the poor, in various forms. Fifthly, who said anything about stopping people leaving?