It has many benefits, including its simplicity. Sure, you could create a framework of incentives and enforcement to make sure only certain people get cash for doing certain things, but those structures will absorb money that might have otherwise gone to the poor. The whole story of international aid, and of UBI if it's done wrong, is a story of propping up inefficient bureaucracies and giving money to aid workers instead of the people who were meant to benefit.
It has many benefits, including its simplicity. Sure, you could create a framework of incentives and enforcement to make sure only certain people get cash for doing certain things, but those structures will absorb money that might have otherwise gone to the poor. The whole story of international aid, and of UBI if it's done wrong, is a story of propping up inefficient bureaucracies and giving money to aid workers instead of the people who were meant to benefit.