From a moral perspective I agree and that should be the long-term goal. Practically, however, the notion of national sovereignty makes it infeasible to implement a global UBI. Within the borders of a country, that country's government can do what is needed to make it work (e.g., punish scofflaws). Across international borders, that's much more difficult. We see this already in that many countries that are "poor" overall have ludicrously high levels of inequality, because a small elite controls all the flows of foreign investment, etc.

To take a simple example: within a country, if you send the UBI money to a local government for disbursement and the local mayor/governor just pockets it, he's still ultimately subject to the legal system of that country and can be jailed, etc. There's no international equivalent of this. If you send some UBI money to Poor Country X for disbursement and the local governor pockets it, there's nothing you can do except not send any more money.