The real reason is because you cannot cleanly define promises when combined with other definitions nor would you neccessarily want to - surprisingly. Say a pol

The real reason is because you cannot cleanly define promises when combined with other definitions nor would you neccessarily want to - surprisingly. Say a politician promises to cut taxes or boost social spending and then a natural disaster strikes or an adversary attacks. Either war or repairs and relief consume time and money. Holding them to it would force suboptimal decisions. Money however is fundamentally fungible and performs best when it flows. Fraud being legal would force far more caution and selectivity which would do vast systemic harm. Why invest if there is no guarantee that it isn't just a gift that may be paid back?