Because soft power is extremely cost effective — convincing people to operate via diplomacy is one thing, but soft power is more, creating good will through charitable acts and expanding on that good will to shape the preferences and opinions of others in ways intended to benefit American policy goals or to prevent catastrophes like epidemics, famine, etc to the best of our ability.
What is most pernicious about Musk is he is screaming fraud and waste about programs that have been allocated support under the law for helping the poorest of the poor around the world, for promoting democracy through cultural exchange, for fighting starvation, malnutrition and disease and for bringing access to vaccines, pharmaceuticals and modern medical treatment to refugees, subsistence farmers, the poor and huddled masses.
It is one thing to disagree with such uses of U.S. resources, especially during a time of deficit spending, but quite another to lie and mislead Americans about what USAID is and what it does so that the richest man in the world can deny any support to some of the poorest and most wretched people across the globe and destroy 50 years of American efforts to build soft power across the globe.