The topical reply is that those positions aren't paying enough.

However, if we have to pay e.g. miners millions to compete with a high UBI, we trigger a massive wage-price spiral. Since extracted raw materials are the bedrock of the entire supply chain, those costs cascade and multiply, eventually making the finished goods unaffordable for the very people receiving the UBI.

In reality, markets don't solve the scarcity of un-fun labor through magic efficiency. They solve it by leveraging debt, poverty, and an exploitable lower class to keep the foundational costs of society artificially low.

Without this DesperationFloor™, the math of our current commodity-based economy falls apart.