That's assuming that everybody calculates utility the same way, which is false. One obvious difference is how to value hypothetical people, which is why there's

That's assuming that everybody calculates utility the same way, which is false. One obvious difference is how to value hypothetical people, which is why there's so much controversy about existential risk charities. Multiple competing utilitarian systems could easily be worse than none at all- eg. the health charities and the AI charities could both calculate the best option is to sabotage each other to avoid "wasting" resources, even if they both agree on expected results of their work. You end up with a prisoner's dilemma type situation. With deontology both "good" and "bad" people are protected by the rule of law, so you avoid that problem. Utilitarianism is ethical anarchy.