Running a charity that you think generates utility less efficiently than some existing charity would be madness.

Many charities could have close marginal worth, and rational allocation of resources would keep them that way. A charity that is less efficient could still perform a useful function, merely needing a decrease in funding, and not disbanding.

And you can't have statically super-efficient charities either, because marginal worth decreases with more funding. For example, a baseline of hundred million dollars SIAI yearly budget might drive marginal efficiency of a dollar donation lower than of other causes.